To determine the effects of circulating arginine vasopressin (AVP) on fetal arterial pressure, venous pressure, heart rate, and blood volume, we infused graded amounts of AVP into chronically catheterized fetal sheep at 122-136 days gestation (term 145-150 days). Plasma AVP concentrations increased by 6.5 +/- 3.9 (SE), 13.0 +/- 2.4, 55 +/- 11.7, and 144 +/- 42 pg/ml with infusion rates of 1, 3.3, 10, and 33 ng/min, respectively. Fetal arterial pressure rose 0, 7.2 +/- 1.0, 9.1 +/- 2.5, 11.4 +/- 2.9, and 12.1 +/- 2.4 mmHg while heart rate decreased 0, 24 +/- 5, 36 +/- 10, 59 +/- 9, and 62 +/- 9 beats/min during AVP infusions of 0.5, 3.3, 10, 33, and 700 ng/min, respectively. Fetal venous pressure was unchanged except that it increased 2.4 +/- 0.5 and 4.8 +/- 1.5 mmHg at infusion rates of 33 and 700 ng/min, respectively. Fetal blood volume remained unchanged except for a 7.2 +/- 3.4% decrease with 700 ng/min. After blockade of the fetal autonomic nervous system, the arterial pressure increase was twice and the heart rate decrease was one-third of those observed in autonomically intact fetuses with comparable AVP infusion rates. Thus these fetal cardiovascular variables showed differential sensitivity to increases in circulating AVP concentrations, suggesting that physiological variations in endogenous AVP may have short-term effects on fetal arterial pressure and heart rate, but probably not on venous pressure or blood volume.