We tested the hypothesis that increased oxytocin is a necessary mechanism for the mediation of androstenedione (delta 4A)-induced myometrial contractions by investigating the effects of maternal treatment with the oxytocin antagonist atosiban on in vivo delta 4A-induced contractions. In four monkeys (group I), maternal estradiol, oxytocin, and myometrial contractions were assessed at baseline and after continuous iv delta 4A administration. Similar measurements were made in three monkeys (group II) that received the same delta 4A infusion regimen, but in addition were treated daily with atosiban. Maternal estradiol and oxytocin levels and contractions were also assessed in four additional monkeys (controls; group III), in which the delta 4A vehicle, intralipid, was infused iv continuously. In group I, delta 4A induced myometrial contractions and increased maternal estradiol and oxytocin to term concentrations. No myometrial contractions occurred in group II monkeys after combined delta 4A and atosiban treatment despite estradiol being elevated to concentrations similar to those measured in group I monkeys. Atosiban had no effect on maternal heart rate or blood pressure. Maternal estradiol, oxytocin, and number of myometrial contractions remained unchanged from baseline values in control monkeys. In conclusion, oxytocin is a necessary part of the mechanisms mediating delta 4A-induced myometrial contractions. delta 4A promotes myometrial contractions via similar mechanisms that mediate spontaneous term contractions in pregnant monkeys.