The effect of hypoxemia on arginine vasopressin (AVP) and oxytocin (OT) release was investigated in the chronically catheterized fetus and ewe. During 30 min of 10% maternal oxygen delivery, mean (±SEM) arterial Po2decreased from 105 ± 10.6 to 48 ± 3.5 mm Hg in the ewe and from 21 ± 1.3 to 12 ± 0.8 mm Hg in the fetus (each P< 0.001). Arterial Pco2decreased from 35 ± 44 to 29 ± 1.0 mm Hg in the ewe, whereas fetal Pco2decreased from 43 ± 2.3 to 35 ± 3.5 mm Hg (P< 0.05). Blood pH increased from 7.44 ± 0.03 to 7.56 ± 0.04 in the ewe (P< 0.01) and from 7.36 ± 0.004 to 7.40 ± 0.006 in the fetuses (P< 0.01). Baseline mean AVP levels were identical in ewes and fetuses (0.7 ± 0.1 µ μ U/ml). After 30 min of hypoxia, plasma AVP levels remained unchanged in the ewes (0.9 ± 0.1), but increased dramatically in the fetuses (47 ± 21 µ μU/ml) (P<0.001). There was a highly significant correlation between the duration of hypoxia and log fetal AVP concentrations (r=0.85). The log fetal plasma AVP also was inversely correlated to the log fetal Po2values (=0.83). Mean baseline fetal and maternal plasma OT levels were 2.6 ± 0.5 µ μU/ml and 2.2 ± 0.5 µ μU/ml, respectively. After 30 min of hypoxia fetal and maternal OT values were 2.9 ± 0.8 µ μU/ml (not significant).