METHODS The animals and diet were as previously described (Kennedy & Mitra, 1963) as were the standardized conditions of temperature and lighting. Experiments were conducted during successive summers to avoid seasonal variations in the time of puberty. Experiment 1. Pairs of litters born on the same day and each consisting of 9-10 pups were mixed, 3 sucklings returned to one mother and the remainder given to the other. When weaned at 21 days the average weight of the optimally fed young was over 40 g, while those reared in large litters weighed between 15 and 25 g. After weaning, both groups were fed ad lib. on MRC Diet 41 B. Only the females were used for further study. The effect of this procedure was studied on (a) somatic growth,(b) sexual development,(c) food intake and (d) sponaneous running activity.

Experiment 2. Twenty-five female rats reared normally in litters of 7-9 were weaned into individual small cages and restricted to a quantity of food which almost stopped growth. They were maintained at their weaning weight for 10 days, then given rather more food so that they increased by approximately 10 g, at which weight they were again held for 10 days, so gaining stepwise until they reached 75 g at 65 days, compared with 45 days for the retarded rats of experiment 1. They were then fed ad lib. Experiment 3. A further 25 rats were treated in the manner of experiment 2, but returned to unrestricted feeding as soon as the vaginas of all of them had opened, when they were 65 g in weight and 55 days old.