Surgical litter restriction to one to five fetuses per rat, results in highly significant increases in fetal body and placental weight. Intrauterine crowding, competition, and the amount of nutrients available per fetus appear to be the limiting factors controlling fetal development. The interaction of ovarian and adrenal hormones was investigated by surgical removal of ovarian and adrenal tissue during litter restriction. Increased intrauterine growth does not appear to be directly controlled by the amount of ovarian and/or adrenal tissue per fetus.