High concentrations of arginine vasopressin (AVP) in arterial umbilical cord blood at the time of delivery have been attributed to either a generalized increase in the activity of the fetal endocrine system at the onset of labor or to fetal asphyxia. We measured AVP in amniotic fluid, arterial and venous cord blood and in maternal venous blood from 13 patients at 38-40 weeks of gestation at the time of elective cesarean section with a nonasphyxic fetus (group I), in amniotic fluid from 19 patients at 15-17 weeks of gestation (group II) and in venous blood from 13 nonpregnant control subjects (group III). Our results showed a high concentration of AVP in the amniotic fluid both in the middle and at the end of normal pregnancy and at the same level as in arterial cord blood, whereas AVP in the venous cord blood was significantly lower and at the same level as in the maternal venous blood and in the control group. It is concluded that the fetus produces AVP and this is at least not solely caused by fetal asphyxia or related to parturition.