The in utero growth rates have been determined for the body, brain, and the placenta in the rat. The accumulation of cell number (DNA content) and protein content in the cerebrum and the placenta were investigated during normal development. When rats were exposed to a severe reduction in protein/calorie intake during the first 10 days of pregnancy and the fetal growth rates were compared to those of the normal control animals, highly significant differences were observed. Pregnant rats, which were restricted in their protein/calorie intake during the early part of pregnancy, significantly prolonged the gestation time. When this increase of in utero time is taken into consideration and the offspring are compared at the time of natural birth, the data presented demonstrate the possibility of in utero catch up growth. The variations, in fetal development within the same litter and across litters, demonstrate a considerable plasticity during normal development in utero.