Chronically prepared fetal lambs had basal plasma arginine vasopressin (AVP) levels of 1.6 +/- 0.23 pg/ml throughout the final third of gestation. Fetal AVP levels measured by radioimmunoassay were increased significantly to 5.8 +/- 0.92, 9.95 +/- 0.95, and 10.0 +/- 5 pg/ml in association with surgical stress, acute exteriorization, and acidosis or hypoxia, respectively. At 59-65 days gestation three lamb fetuses had a 50-fold increase in plasma AVP levels after acute exteriorization and hemorrhage indicating that the 0.4 gestation lamb fetus is capable of secreting pituitary AVP stores after intravascular volume change, temperature stress, and tactile stimulation. In six chronically catheterized third trimester fetuses, a 10-min, 20% hemorrhage caused a significant fall in fetal blood pressure, heart rate, and hematocrit. Plasma AVP increased significantly to 24.3 +/- 5.8 ng/ml without change in arterial blood gases or pH. Basal plasma prolactin concentration of 9.0 +/- 2.0 ng/ml was unchanged after hemorrhage in five fetuses.