Thirteen Fischer strain rats were divided randomly into 3 groups soon after became pregnant and fed a 10%, 18%, or 36% casein diet throughout pregnancy and lactation. At weaning, female pups from the dams were divided into 3 further groups which were fed throughout life 10%, 18%, or 36% casein diet. The animals were weighed regularly and given a full autopsy after death. DNA was determined in the cerebrum and cerebellum of progeny at 7, 15 and 50 weeks of age respectively. The level of protein intake of the dams did not affect the litter-size, but did affect the body weight of pups at weaning. When fed a high protein diet after weaning, pups from dams fed a low protein diet weighed less throughout life than those from dams fed a high protein diet. A similar effect of protein intake was observed on the tail length. Pups of dams fed a low protein diet ate a larger amount of diet per unit body weight after weaning than those of dams fed at a high level of protein. Pups at 7, 15, and 50 weeks of age respectively from dams fed 10% casein diet had a lighter cerebrum and cerebellum than pups of the same ages from dams fed respectively 18% and 36% casein diet. They also had less total DNA in the cerebrum and cerebellum than the latter. At the ages of 1 and 2 years respectively, the survival rates of rats fed the same diet as their mothers tended to be greater than those of rats fed diet different from that of their mothers. They also tended to live longer than the latter: 4 of 60 rats lived for over 1,000 days, and 3 of these were fed the same diet as that of their mothers. Lesions observed at death did not differ in different groups. However, the incidence of lesions of the kidney and the number of tumors seemed to be greater in groups fed a high protein diet.