The effect on reproduction and fetal growth of a protein-deficient diet administration during pregnancy was studied in young and adult rats. Young (50-55 days old) and adult (90-100 days old) pregnant or nonpregnant rats were fed a normal diet (25% protein) or a protein-deficient diet (6% protein) during pregnancy or for a 22-day period (nonpregnant rats). All females were weighed during the experiment and body length measured in the young rats. After parturition, pups were counted, sexed and individually weighed. Litter size, number of stillbirths and presence of body lesions in the neonates were also recorded. Alimentary protein deficiency caused reduction in weight gain during pregnancy and in the postpartum period in young and adult rats. Pups from protein deficient dams weighed less at birth than the pups of control dams, although litter size was unaltered. Pups from young malnourished dams tended to weigh less than those from adult malnourished dams. The incidence of stillbirths was higher in malnourished rats, the highest values occurring in the adult group. These results suggest that alimentary protein deficiency during pregnancy in young rats reduces maternal weight gain, presumably reducing nutrient storage. This may cause fetal/maternal competition for nutrients leading to retardation of both maternal and fetal growth. Growth impairment may be an adaptive process, assuring fetal survival.