Three groups of mature Merino ewes, balanced for body weight and number of ovulations, and totalling 122 animals, received the following treatments subsequent to mating to fertile rams: 1. approximately half maintenance requirements from day 7 to 37; 2. approximately half maintenance requirements from day 7 to 37, plus 10 mg progesterone daily from day 20 to 37; 3. control.

The ewes, which were in fat condition, had a mean ovulation rate of 121·3 %; except during the submaintenance period, they gained weight throughout pregnancy.

The percentages of ova not represented by lambs at term were 34·7, 46·0 and 42·6 in groups 1 to 3 respectively, the differences being non-significant. Detailed examination of the cycle length data of ewes returning to service suggests that some embryos may have been killed by the submaintenance treatment. Progesterone treatment was not beneficial, nor did its sudden withdrawal on day 37 appear to be detrimental to foetal survival. In a sample of ewes slaughtered on day 37, adrenal gland weights were significantly lower in the two treated groups.

Significantly higher losses of ova occurred among twin than among single ovulators and there was some evidence that this was non-random. There was no significant relationship between prenatal mortality and body weight at mating.