SummaryMaternal and fetal plasma concentrations of arginine vasopressin (AVP) during asphyxial and hypoxemic episodes were ascertained between 130 and 140 days of gestation in chronically catheterized sheep. During an acute asphyxial stress,i.e., decreased PaO2and pHa and increased PaCO2, maternal AVP in plasma was unaltered, whereas fetal arterial plasma concentrations rose from 1.6–2.2 μ/ml to 34–385 μ/ml and were associated with massive expulsion of meconium into the amniotic fluid. Mild hypoxemia, induced while the mother breathed a gas mixture consisting of 85% nitrogen and 15% oxygen, did not affect either maternal or fetal plasma AVP concentrations. The use of 10% inspired oxygen resulted in 60% and 50% reductions in maternal and fetal PaO2, respectively (P < 0.05). In this instance, the maternal plasma AVP levels were unchanged, whereas the fetal plasma AVP concentration rose from a mean of 2.61 ± 0.14 (SE) to 10.2 ± 2.59 μ/ml (P < 0.025) within 30 min. Expulsion of meconium into the amniotic fluid did not occur. No evidence of either fetal-maternal placental transfer or fetal-placental clearance of plasma AVP was obtained. Although hypoxemic stress resulted in an elevation of fetal plasma AVP concentration, it does not appear to be the sole factor responsible for AVP release during intrauterine stress. It is suggested that substantial elevations in fetal plasma AVP concentrations may play an integral role in the fetal expulsion of meconium into the amniotic fluid.