Physiological and behavioral development of rats was affected by prenatal nutrition and postnatal litter size. Prenatal nutrition was manipulated by combining differences in maternal nutrition with variation of prenatal litter size produced by pre-mating isolation of 1 uterine horn. The pups were reared in postnatal litters of either 4 or 12. During the 1st postnatal week, development and free behavior were affected only by the prenatal treatments. During the 2nd postnatal week, both prenatal and postnatal effects were observed. Development and behavior during the 3rd postnatal week reflected only postnatal litter size. Brain weight at weaning was influenced by both the prenatal and postnatal treatment.