1. Male hooded Lister rats given a diet containing 40 mg zinc/kg were described as Zn-adequate. Other rats were subsequently given diets containing less than 1 mg Zn/kg. After a period of approximately 5 d these animals ceased to grow and were described as Zn-deficient.
2. Zn-deficient rats offered ad lib. a Zn-deficient diet containing 200 g egg albumen/kg ate only 55% of the weight eaten by Zn-adequate rats given a similar diet supplemented with Zn. The intake of the deficient rats increased when the metabolizable energy content of the diet was decreased and also when the environmental temperature was lowered.
3. Zn-deficient rats offered Zn-deficient diets containing 200 g egg albumen/kg showed a high day-to-day variability of intake. When the albumen content was raised to 400 g/kg, neither the mean food intake of the rats nor the variability of food intake changed, but with diets containing only 50 g albumen/kg the quantity eaten increased and the variability of food intake decreased. Results obtained when the low-protein diet was supplemented with essential and non-essential amino acids indicated that increased variability of intake was associated with the essential amino acid content of the diet. The effect on variability of intake was greatest when the supplements contained methionine, phenylalanine, threonine and tryptophan; addition of this group of amino acids to a Zn-supplemented, low-protein diet produced the largest increase in the growth rate of Zn-adequate rats.
4. When the food intake of the rats was examined for periods of 2 h throughtout the day, the Zn-deficient rats were found to eat on fewer occasions than the control rats. However, in those periods when the Zn-deficient rats did eat, the quantities eaten in 2 h showed the same distribution of weights as did those for the Zn-adequate rats.
5. There were significant relationships between food intake and plasma Zn concentration; the most significant was the negative correlation between food intake in 24 h and plasma Zn concentration at the end of the 24 h period.
6. Zn deficiency resulted in a failure of growth in the young rat and therefore in a reduction in its rate of energy expenditure but did not appear to cause directly a loss of appetite. It is suggested that cyclical patterns of food intake associated with Zn deficiency in young rats resulted from the slow but effective control of food intake by the energy balance of the animals.