The long-range effects of ligating one umbilical artery were studied in 12 lamb fetuses using chronic, implanted fetal arterial catheters. Their physiologic adjustments were compared to 14 control fetuses with intact umbilical circulation. The control fetuses reached normal arterial pH values within 1–2 days, whereas the umbilical artery ligated group did not reach normal pH values until 3–4 days, thus supporting the presence of a placental reserve capacity. In 7 fetuses who survived for more than 19 days with a ligated umbilical artery, malnutrition and retarded growth were observed in the presence of normal arterial blood pH and near normal arterial pO2values. In these malnourished fetuses, we observed a preferential continued development of the brain, heart, spleen and kidneys, but the liver, lungs and thymus failed to grow. Possible mechanisms of fetal adaptation arc discussed.