There is presently no information concerning the ontogeny and control of arginine vasopressin (AVP) in the human fetus. AVP was measured in 22 nonanemic control fetuses and 7 fetuses with hemolytic anemia undergoing 13 intravascular transfusions. Each transfused fetus received pancuronium (0.3 mg/kg) and furosemide (2 mg/kg). Compared to the control group of nonanemic fetuses with hemolytic disease, AVP was significantly lower in the anemic fetus prior to transfusion (2.6 ± 0.4 μU/ml versus 10.4 ± 4.1 μU/ml, p < 0.05). This suggests that hemolytic anemia is associated with a relative increase in fetal intravascular volume. Intravascular transfusion was associated with a significant increase in AVP (p < 0.05). These findings could not be explained by changes in either blood pressure, plasma osmolality, or fetal oxygenation.