1. Casein was labelled with pairs of radioactive amino acids, lysine, tyrosine and leucine, one with I4C and the other with 3H, by jugular infusion into lactating goats followed by isolation of the double-labelled casein from the milk. Total milk protein was similarly labelled by jugular infusion of [35S]cystine. U-14C-labelled fraction- 1 leaf protein was isolated from lucerne (Medicago sativa) grown in an atmosphere of 14C02

2. The proteins were treated withdifferent levels(333 and667 mmol/kgprotein) offormaldehyde, glutaraldehyde and glyoxal.

3. Absorption from the small intestine was measured in sheep with fistulas in the abomasum and terminal ileum, using Cr-EDTA as the digesta flow marker, by introducing radioactive casein into the abomasum.

4. Lysine, tyrosine and cystine became increasingly unavailable for absorption from the small intestine of sheep with increasing levels of aldehyde. At the lower level (333 mmol/kg) the proportions of the amino acids that were unavailable were 0.192, 0.051 and 0.123 respectively. At the higher level of formaldehyde (667 mmol/kg) the corresponding values were 0.335, 0.201 and 0.432 respectively. Leucine was not made unavailable with formaldehyde.

5. The proportions of lysine, tyrosine and leucine that were unavailable were higher, on a molar basis, after treatment of the proteins with the dialdehydes glutaraldehyde and glyoxal than after treatment with formaldehyde. However, the extent of protein protection provided by the dialdehydes in the rumen, measured using an in vitro procedure, was lower.

1. Four pigs, initially of 30 kg live weight, were surgically prepared with two re-entrant cannulas in the jejunum, 1.0 m apart. This allowed an isolated loop to be formed through which Ringer solutions were continuously perfused for 6.5-h periods per day.

2. The effects of two Ringer solutions of contrasting composition on nitrogen secretion were measured in a preliminary study.

3. The effects of guar gum (6.7 g/l) on N secretion during perfusions of solutions of glucose or of maltose (20 g/l) were measured.

4. N secretion did not differ significantly between the two Ringer solutions.

5. Addition of guar gum to the solution of glucose increased mean N secretion from 69.2 to 133.9 mg/m per 2 h; the corresponding values for the maltose solution were 75.5 and 120.2 mg/m per 2 h. In both cases the differences were significant (P< 0.001). Differences between N secretion into glucose and maltose solutions were not significant, either without or with guar gum.

6. Guar gum addition had a greater effect on N secretion in hours 5 and 6 than in hours 1 and 2 of perfusion.

7. It was calculated that guar gum would have increased N secretion by the mucosa of the whole small intestine from approximately 15 to 27 g/d under the conditions of the study, assuming uniform secretion throughout the organ. These results suggest that certain types of dietary fibre may be important determinants of N secretion by the mucosa of the small intestine. They also suggest that changes in N secretion of this magnitude are of importance in N metabolism because the overall rate of protein synthesis in these pigs was probably about 100 g/d (expressed as N), using values from the literature.

1. Studies have claimed that an enhancement of the thermic effect of a meal (TEM) is an important adaptive mechanism to account for energy wastage during over-feeding.

2. Eight healthy normal-weight young men were studied during 1 week on a weight-maintenance diet and again during 1 week when they were over-fed by 50% with fat. During each experimental week, the subject occupied a whole-body indirect calorimeter at 26° for two separate periods of 36 h. The periods differed in the amount of exercise they contained. The thermic responses to the identical meals were measured during rest on one occasion and during exercise on a bicycle ergometer on the other.

3. On the maintenance diet the absolute TEM (kJ/min) was 1.51 (SD 0.42) at rest and 1.31 (SD 0.75) during exercise (no significant difference). The equivalent values (kJ/min) on the over-feeding diet were 2.2 (SD 0.48) and 1.97 (SD 0.64) (no significant difference).

4. The absence of a significant interaction between TEM and exercise was also demonstrated by the fact that the effect of over-feeding on total 24 h energy expenditure was unaffected by the subject's level of physical activity while in the calorimeter.

5. In conclusion, the present study has provided no evidence to support the hypothesis that TEM is enhanced during exercise.

1. Three steers, each fitted with a rumen cannula and a re-entrant cannula in the proximal duodenum, were offered diets consisting of a barley-based concentrate and chopped hay at a daily intake of 61 g/kg live weight0-75 given in three. equal meals. The ratio, concentrate: hay was changed from 50: 50 to 90: 10 and then to 100: O in successive periods of 12–18 weeks and the flow and composition of digesta at the duodenum was measured over 48-h periods on each dietary treatment.

2. Samples of bacteria and protozoa were separated from rumen contents and the proportions of bacterial and protozoal nitrogen (N) in duodenal digesta were estimated using 2, ddiaminopimelic acid (DAPA) and 2- aminoethylphosphonic acid (AEP) as markers. On separate occasions, radioactive sulphur (35S) was infused into the rumen for 48 h and digesta collected over the final 24 h; the specific radioactivity of S in microbial and digesta fractions was used to estimate the proportions of microbial N.

3. 35S gave reproducible and apparently reliable estimates of microbial protein formation: the proportion of microbial N in digesta was significantly higher (P < 0.05) for the 50:50 diet than for the other treatments but the energetic efficiency of microbial protein formation did not differ significantly between diets.

4. Estimatesof bacterial N based on DAPA concentrations were highly variable and frequently impossibly high. It is suggested that many of the anomalous values were the result of non-representative sampling of the rumen microbial population and that this is particularly likely to occur when conditions within the rumen are unstable. AEP was found to be unsuitable as a marker for rumen protozoa as considerable concentrations of this substance were found also in rumen bacteria.

1. Twelve steers fitted with rumen cannulas were used in three separate experiments to investigate the effects of the presence or absence of rumen ciliate protozoa on methane production. The diet consisted of 850 g barley and 150 g protein supplement/kg, and was given in three feeds daily at a restricted level of 61 g/kg live weight0.75. Animals were defaunated initially by allowing ad lib. consumption of this diet and were then maintained ciliate-free by isolation or were faunated by inoculation with a mixed ciliate suspension. Samples of rumen fluid were taken routinely for the assessment of microbial populations and for volatile fatty acid (VFA) analysis and energy and nitrogen balances and digestibility measurements were made at intervals while animals were confined in respiration chambers.

2. In each experiment the rumen VFA proportions changed from a high-propionate pattern under ciliate-free conditions to a low-propionate, high-butyrate pattern in the presence of ciliates: differences between treatments were highly significant (P < 0.001). There were also marked differences between treatments in CH, production but a reliable comparison was possible only in Expt 3, in which CH4 was significantly higher (P < 0.001) in the presence of a rumen ciliate population. In Expt 3 the increased loss of energy as CH4 in the faunated animals amounted to 44 MJ/100 MJ energy intake.

3. Stoichiometric estimates of CH4 production derived from the observed VFA proportions showed good agreement with CH4 production as measured in respiration chambers. On average, the stoichiometric CH4 values overestimated CH4 production by a factor of 1.08.

4. Highly significant linear relationships (P < 0.001) were observed between the molar proportion of each major VFA and the quantity of CH4 produced: the proportion of propionic acid was inversely related to CH4 and showed the lowest residual standard deviation of all the relationships examined.

5. The losses of energy in faeces and urine did not differ between treatments hence the increased loss of energy as CH4 in the faunated animals resulted in a significant reduction in the metabolizability of the diet from 0.73 to 0.69 (P < 0.05). No significant differences were detected between treatments in heat production, apparent digestibility coefficients or N balance.

6. It is suggested that the rumen ciliates, by modifying the rumen VFA proportions, are directly responsible for the increased CH4 production in faunated animals.

1. The aleurone layer, grain coat and embryo which constitute rice bran are rich in vitamins, lipids, protein and lysine compared with the endosperm (milled rice).

2. A method was developed to measure the in vitro protein digestibility of raw and cooked brown rice and their histological components.

3. The protein digestibility of cooked endosperm by the in vitro method agreed with that of other workers using in vivo techniques.

4. The protein digestibility of the aleurone layer and grain coat from raw rice was only 25% but increased to 65% from cooked rice, due to disruption of the cellulosic cell walls at 100°, which was shown by electron microscopy.

5. The decreased protein digestibility due to cooking was not the result of formation of ε-lysyl–γ-glutamyl isopeptide cross-links, but may be due to formation of a cystine-rich core that is resistant to proteases.

6. The protein digestibility of cooked brown rice was approximately the same as that of cooked milled rice, hence it is advantageous for those for whom rice is a staple food to consume brown rather than milled rice.

1. The chromium concentration was determined in 261 samples of breast milk collected by manual expression from forty-five American women.

2. Cr was measured in whole, liquid milk by graphite-furnace atomic absorption spectrophotometry, using the method of standard additions.

3. The mean Cr content of the breast milk samples was 0.30 ng/ml. The range of individual values was 0.06–1.56 ng/ml and did not change significantly with duration of lactation.

4. The Cr content of breast milk from these American women is similar to levels reported recently in milk from Finnish women.

1. Testosterone (1 mg/kg body-weight per d) given subcutaneously to female rats increased their growth rate and food conversion efficiency but not their food intake compared with that of the placebo-oil controls. A higher dose of testosterone (10 mg/kg body-weight per d) failed to increase the daily weight gain.

2. The increased growth rate of the testosterone-treated rats appeared to occur in the whole body and not specifically in muscle. There were no significant changes in body composition.

3. The fractional synthetic rate of gastrocnemius muscle protein was higher in the hormone-treated rats than in controls. This contrasts with previous results for the anabolic agent trenbolone acetate and reflects differences in the mode of action of these two different steroids, both with androgenic properties.

1. Female Wistar rats were given an adequate-zinc (60 μg/g) or low-Zn (7 μg/g) diet for a minimum of 2 weeks and then mated. They were then either continued on the same diets (+Zn –Fe or –Zn –Fe) or given similar diets supplemented with four times the normal level of iron (+Zn + Fe or –Zn + Fe). The day before parturition they were killed and the fetuses removed and analysed.

2. There were no differences in numbers of fetuses or the number of resorption sites. In the absence of Fe supplementation, the mean fetal wet weight was significantly less (P < 0.05) in the low-Zn group but there was no effect of Zn in the two Fe-supplemented groups. The addition of Fe significantly decreased (P < 0.05) the mean fetal wet weight in the adequate-Zn groups but had no effect in the low-Zn groups. There were no differences in fetal dry weight, fat, protein or DNA content. Both Fe-supplemented groups produced fetuses of higher Fe concentration (P < 0.01), and mothers with higher bone Fe-concentration (P < 0.01) compared with the non-supplemented groups. The low-Zn groups produced fetuses of lower Zn concentration (P < 0,001) than the adequate-Zn groups but there was no effect on maternal bone Zn concentration.

3. It was concluded that Fe-supplements did not adversely affect fetal growth from mothers given a low-Zn diet, but the addition of Zn to the unsupplemented diet increased fetal wet weight. These findings were not accompanied by any other differences in fetal composition or dry weight, and do not therefore lend support to the suggestion of an Fe-Zn interaction.

1. The metabolism of glucose has been studied in 48-h-old unanesthetized fasting and post-absorptive sucking piglets.

2. Both [6-3H]- and [U-14CJglucose were administered either by a single injection method or by a primed infusion technique. The rates of glucose turnover and recycling were estimated under steady-state conditions.

3. The rates of glucose turnover and recycling in 48-h-old fasting or post-absorptive piglets were not statistically different when measured using the single injection technique or the printed infusion method.

4. The mean (with SE) rate of glucose turnover was 65.8 (2.5) in post-absorptive and 31.1 (1–9), μ mol/kg per min in fasted newborn pigs. Glucose utilization was linearly related to blood glucose concentration; regression analysis indicated a y-intercept of 7.2, μ mol/kg per min.

5. As tested by arterio-portal differences the gut was not releasing glucose or galactose in 5 h-post-absorptive sucking newborn pigs. Thus, the higher rates of glucose turnover in post-absorptive newborn pigs compared with fasting ones suggest that hepatic glucose production is enhanced in post-ahsorptive sucking piglets.

6. The mean (with SE) rates of glucose recycling were four times higher in post-absorptive piglets than in tasting ones, i.e. 14.4 (1.6) and 3.7 (0.5)% of [6-3H] glucose turnover respectively. As liver glycogen was exhausted in 48-h-old sucking piglets, this suggests that hepatic glucose production results from gluconeogenesis.

1. The effects of magnesium Supplementation on Mg absorption proximal and distal to the pylorus in ewes maintained on a grass diet were investigated using a combination of balance, digesta flow and electropotential Measurements.

2. Three mature ewes each received by intrarumenal infusion a supplement of 0, 1, 2 and 3g Mg/d in sequence over four 10-d periods.

3. Net Mg absorption distal to the pylorus took place down its electrochemical gradient, although the quantity absorbed remained small during the control and first infusion periods.

4. The bulk of Mg absorption occurred before the pylorus and, during the control and first infusion periods, took place against its electrochemical gradient. The net Mg absorption proximal to the pylorus rose with declining efficiency as Mg intake was increased. It is suggested that saturation of the absorption process at this site was occurring.

1. In two separate trials male and female Wistar rats, 12 weeks of age, were either killed as a preliminary control group, ad lib.-fed or undernourished for 4 weeks until one-third of their 12-week body-weight was lost.

2. Food intakes, urinary and faecal collections and measurements of standard metabolic rate were made at one-weekly intervals on both the ad lib.-fed and undernourished animals of both sexes.

3. The bodies of the preliminary controls, the ad lib.-fed and the undernourished animals of both sexes were analysed for protein and fat, and the weights of four fat depots, two muscles and the major organs of all groups were determined.

4. Measurements of lipid synthesis rate (LSR) and lipoprotein lipase (EC 3.1.1.34) (LPL) activity in the four fat depots and measurements of whole-body protein synthesis rates were carried out on animals of both sexes in each group.

5. Although both sexes lost the same proportion of body-weight the females required more food on a body-weight basis than the males during the undernutrition period. The females absorbed significantly more energy on a body-weight basis during undernutrition and so were less efficient than the males at withstanding nutritional stress.

6. There were no significant differences between males and females, on a body-weight basis, in the excretion of nitrogenous waste products (urinary nitrogen, creatinine, hydroxyproline or NT-methylhystidine) suggesting that there were no differences between the sexes in protein sparing during undernutrition.

7. There were no differences between males and females in the proportions of body fat and protein used during the period of undernutrition or in the sites of the body from which the protein and fat were mobilized.

8. There were no differences between males and females in the way they responded to undernutrition by altering LSR, LPL activity or whole-body protein synthesis rates. Both undernourished males and undernourished females maintained synthesis of lipid, on a per g tissue basis and whole-body protein synthesis at the level found in well-nourished animals of the same sex.

1. The intake of mammary secretion from delivery to day 5 post-partum was determined by test-weighing nine infants using an integrating electronic balance. The mean yield of colostrum for the first 24 h after birth was 37.1 (range 7.0–122.5) g and was 408 (range 98.3–775) and 705.4(range 452.5–876)g/24h on days 3 and 5 post-partum respectively.

2. The milk yield of mothers on either day 14 or 28 post-partum was determined by test-weighing the mother. The mean milk yield was 1.156 (SD 0.167) kg/24 h.

3. A significant correlation (P < 0.001; r 0.85, n 42) was found between milk yield measured by test-weighing the infant and milk yield measured by test-weighing the mother, confirming that it is possible to obtain a similar estimate of milk consumed using either of the two methods of test-weighing

4. There was a significant positive correlation (P < 0.001) between lactose concentration and milk yield for the first 5 d post-partum (r 0.76, n 22); a significant negative correlation (P < 0.001) between protein concentration and milk yield (r – 0.74, n 22) and no significant correlation between fat concentration and milk yield for the period studied.

5. The calculated energy intake of infants during the first 24 h after birth was only 0.12 (range 0.02–0.29) mJ. This increased to 1.44 (range 0.83–2.18) and 2.99 (range 2.49–4.06) mJ/24 h by days 3 and 14–28 post-partum respectively.

1. Female Wistar rats (5 and 11 weeks old) were either left intact or ovariectomized. Animals of each age- and treatment-group were either ad lib.-fed or undernourished for 4 weeks.

2. The bodies of all animals were analysed for protein and fat and the weights, lipid synthesis rate and lipoprotein lipase (EC 3.1. 1.34) activity of four fat depots were determined.

3. The well-nourished ovariectomized animals of both age-groups gained weight more rapidly than the well-nourished intact animals of the same age, but there was no effect of ovariectomy on body composition, lipid synthesis rate or lipoprotein lipase activity in either the well-nourished or the undernourished animals of either age-group.

4. There was a greater efficiency of energy utilization found in the ovariectomized animals than in the intact animals regardless of age or nutritional status.

1. Estimates have been made of the irreversible loss of alanine and of glycine in chronically catheterized fetal lambs and in sucking lambs using [U-14C]-labelled radioisotopes. The experiments in the fetal lambs were carried out at least 5 d after implantation of catheters.

2. The mean concentration of glycine in fetal femoral arterial blood between 102 and 129 d conceptual age was 755 μmol/l and this was not significantly different from that in maternal venous blood. The mean concentration of alanine in fetal femoral arterial blood during the same period of gestation was 229μmol/l and this was significantly greater than that in maternal venous blood.

3. Assuming a catenary model, the mean irreversible loss of glycine, determined using the single-injection technique, in three fetal lambs of 107, 111 and 127 d conceptual age was 17 μmol/min per kg, whereas in two fetal lambs aged 106 and 109 d into which the isotope was infused continuously the mean irreversible loss, calculated from the specific activity of glycine 5 h after the start of infusion of the tracer ('pseudo plateau'), was 12 μmol/min per kg. In a sucking lamb, 9 d after birth, the irreversible loss of glycine was 11 μmol/min per kg. The mean irreversible loss of alanine, determined by the single-injection technique assuming a catenary model in five fetuses between 112 and 121 d conceptual age was 14μmol/min per kg, and in two sucking lambs, 9 and 11 d after birth, it was 5.1 μmol/min per kg.

4. When a two-pool model was assumed in which entry of metabolite was not directly into the sampling pool but was by way of the second pool, then the mean irreversible loss of glycine in the three fetuses was 23 μmol/min per kg and of alanine in the five fetuses was 32 μmol/min per kg. Calculations based on the alternative two-pool model did not alter appreciably the rates of irreversible loss of either alanine or glycine in the sucking lambs.

5. From a comparison of the specific activities of the amino acids and of carbon dioxide in blood during the course of the experiments, it was found that in the fetuses 0.96% of the CO2 present in blood was derived from alanine and only 0.12% was derived from glycine. It was calculated that not more than 1.6 μmol lanine/min per kg and 0.29 μmol glycine/min per kg could have been converted to CO2 in the fetal lambs.

6. It is concluded that since glycine in fetal blood originates from fetal tissues and not from direct transfer across the placenta the upper value for the irreversible loss describes metabolism best. In the case of alanine, which is derived from both the maternal circulation and from metabolism in fetal tissues, the true rate of irreversible loss must lie between the values predicted by the two models.

1. The effect of a palatable granulated guar-gum preparation (10 g twice daily) was studied in obese subjects. The acute effect of a single dose of guar gum to reduce the peak postprandial whole blood glucose levels (about 10%) was verified. Following long-term treatment, a further reduction was seen in the obese subjects with the highest postprandial glucose levels. Since the postprandial plasma insulin levels were essentially unchanged, this finding suggested an increased responsiveness to insulin. Total serum cholesterol levels were significantly reduced following long-term treatment but serum α-cholesterol levels, representing the high-density-lipoprotein fraction, was unchanged.

2. Body-weight was significantly reduced during guar-gum treatment even though the patients were asked to maintain their normal dietary habits. Daily hunger ratings recorded for up to 10 weeks showed that guar gum reduced hunger significantly better than commercially available bran taken in the same way.

3. Thus, guar gum seemed to influence carbohydrate and lipid metabolism in a beneficial way in obese subjects. The reduction in hunger would offer an additional benefit to these patients.

1. Male weanling rats were fed on diets either adequate (55 mg/kg), or severely deficient (0.4 mg/kg) in zinc, either ad lib. or in restricted amounts in four experiments. Measurements were made of growth rates and Zn contents of muscle and several individual tissues.

2. Zn-deficient rats exhibited the expected symptoms of deficiency including growth retardation, cyclic changes in food intake and body-weight.

3. Zn deficiency specifically reduced whole body and muscle growth rates as indicated by the fact that (a) growth rates were lower in ad lib.-fed Zn-deficient rats compared with rats pair-fed on the control diet in two experiments, (b) Zn supplementation increased body-weights of Zn-deficient rats given a restricted amount of diet at a level at which they maintained weight if unsupplemented, (c) Zn supplementation maintained body-weights of Zn-deficient rats fed a restricted amount of diet at a level at which they lost weight if unsupplemented (d) since the ratio, muscle mass:body-weight was lower in the Zn-deficient rats than in the pair-fed control groups, the reduction in muscle mass was greater than the reduction in body-weight.

4. Zn concentrations were maintained in muscle, spleen and thymus, reduced in comparison to some but not all control groups in liver, kidney, testis and intestine, and markedly reduced in plasma and bone. In plasma, Zn concentrations varied inversely with the rate of change of body-weight during the cyclic changes in body-weight.

5. Calculation of the total Zn in the tissues examined showed a marked increase in muscle Zn with a similar loss from bone, indicating that Zn can be redistributed from bone to allow the growth of other tissues.

6. The magnitude of the increase in muscle Zn in the severely Zn-deficient rat, together with the magnitude of the total losses of muscle tissue during the catabolic phases of the cycling, indicate that in the Zn-deficient rat Zn may be highly conserved in catabolic states.

1. Incubation of hepatic microsomes with retinol resulted in formation of retinyl esters and glucuronides. The presence of the cytosolic lipid-protein aggregate (LPA) in the system in addition induced release of holoretinol-binding protein from the microsomes. The extent of these reactions was influenced by the addition of coenzyme A and ATP, or uridine diphosphate glucuronic acid.

2. Incubation of hepatic microsomes containing labelled retinyl esters with the LPA resulted in the appearance of the labelled retinyl esters in the LPA.

3. Small amounts of retinoic acid were formed on incubation of retinol with microsomes (approximately 1% of added retinol); this was found to be associated with a protein of approximately 14500 molecular weight, and less than 10% was associated with the LPA. This is in contrast to retinol, which was found to be almost completely associated with the LPA.

4. The cytosolic LPA was associated both with carotene-cleavage activity and alcohol dehydrogenase (NAD(P)+) (EC 1. 1. 1. 71) activity.

5. These findings lend some support to the concept of a specific role for hepatic LPA.

1. Four mature Friesian cows were used in a Latin square design experiment to measure the effects of abomasal casein infusion on milk production and the kinetics of glucose, acetate and palmitate in jugular venous blood.

2. The basal ration contained 110 g crude protein/kg dry matter and was offered at the rate of 4 kg hay, 3 kg alkali-treated straw cubes plus 9 kg concentrates/d. Treatments were infusion of casein into the abomasum at the rate of 0, 240 or 460 g/d. The experiment started 1 week post partum. Experimental periods lasted 21 d.

3. [6-3H]glucose, [9, 10-3H]palmitate and [U-14C]acetate were continuously infused into one jugular vein for measurements of whole-body flux rates.

4. The yield of milk protein was increased significantly (P < 0.05) by the first increment of casein infusion, with no further increases.

5. Casein infusion had no effect on blood concentrations of glucose, acetate, palmitate or growth hormone. Mean flux rate of glucose increased with each increment of casein. The effect at 460 g/d was higher (P < 0.1) than with no infusion. Flux rates of palmitate and acetate were increased (P< 0.1) by the first increment of casein. The proportion of blood bicarbonate derived from acetate increased (P < 0.05) with the first increment of casein.

6. Protein status of cows early in lactation influences the dynamics of glucose and fatty acid metabolism.

1. The effects of vitamin D3 (D3) on serum levels of 1, 25-dihydroxyvitamin D3 (1, 25(OH)2D3), ionic calcium, total Ca and phosphorus in chicks were studied from the time of hatching until sexual maturity.

2. Chicks fed on a diet low in D3 showed a serum level of 1, 25(OH)2D3 higher than that in chicks on a normal-D3 diet, for both sexes and at any given age.

3. A dramatic increase in the serum level of 1, 25(OH)2D3 occurred in female birds approaching sexual maturity and in laying hens raised on the low-D3 diet the level was five times that of their counterparts raised on a normal-D3 diet.

4. Theserum 1, 25(OH)2D3levelin adultmalesin thelow-D3groups wasseven timesthatofthoseon thenormal-D3 diet.

5. The serum level of 25-hydroxyvitamin D3 remained relatively unchanged at weeks 2 and 15 in birds on a low D3 intake as well as in those fed on a normal-D3 diet. Nevertheless, the levels of 25-hydroxyvitamin D3 were different between the two groups.

6. No significant change was observed in the level of ionized serum Ca in relation to dietary regimen, but there was an increase in total Ca concentration in females with the onset of reproduction.

7. The serum P level decreased gradually with age, reaching a minimum value 3 and 8 weeks before laying commenced in the groups on low- and normal-D3 diets respectively. An increase was observed when the hens began laying.

8. Chicks adapted to a low-D3 diet by elevation of their plasma level of 1, 25(OH)2D3. The mechanism by which this is achieved is not known, but the results suggest that parathyroid hormone, Ca and P are unlikely to play roles in the adaptive increase in the level of 1, 25(OH)2D3 in the blood of chicks given a minimal amount of D3. The possibility that the rate of degradation of 1, 25(OH)2D3 is greatly reduced under these conditions cannot be excluded and this could account for the level of this metabolite in those birds.