The purpose of the present study was to determine whether endogenous atrial natriuretic factor participates in the maintenance of normal vascular pressure and renal function in ovine fetuses at 128 to 130 days' gestation. Circulating atrial natriuretic factor in the fetus was immunoneutralized by an intravenous bolus injection of an atrial natriuretic factor antiserum at a dilution of 1:2000 (low dose, n = 7) or 1:400 (high dose, n = 6). In the high-dose group, plasma atrial natriuretic factor concentration was significantly reduced by 65 ± 14 pg/ml from basal levels of 165 ± 12 pg/ml within 10 minutes and remained reduced for the 90-minute period after the injection. Fetal arterial pressure acutely and transiently decreased, but at 50 minutes arterial pressure increased and was elevated for the remainder of the experiment. Urine flow and urinary excretion rates of sodium, potassium, and chloride were reduced within 10 minutes after the injection. Urine flow rate was suppressed for as long as plasma atrial natriuretic factor concentrations were reduced. Fetuses in the low-dose and control groups showed no significant change in cardiovascular or renal function. In response to atrial natriuretic factor antiserum injection, plasma angiotensin II concentrations were increased, whereas plasma arginine vasopressin concentrations were unchanged. These results suggest that endogenous atrial natriuretic factor is involved in the maintenance of arterial pressure and urinary excretion in the ovine fetus.