Normal term labor is associated with a surge in myometrial oxytocin receptor formation and gap junction development. We have previously shown that inhibition of prostaglandin synthesis by naproxen sodium, 2.0 mg/day, suppressed oxytocin receptor formation but not gap junction formation and prolonged gestation. In this study, we investigated the effects of a specific oxytocin antagonist on oxytocin receptor formation, gap junction formation, and labor in the rat. [Pen1Phe(Me)2,Thr4,Orn8]oxytocin, a specific oxytocin antagonist, was infused subcutaneously during the last 3 days of pregnancy at 300 μg/day. Measurements of myometrial oxytocin receptor concentrations and gap junction formation on days 21 and 22 and days 22–23 (in labor) pregnant uteri showed no significant differences in the Bmax and Kd values between the control and the treated group. Gestation period was not prolonged by the oxytocin antagonist. However, in a separate group of day 23 pregnant rats, the uterine contractile response to 60 mU of oxytocin i.v. was found completely blocked by 10 μg of the oxytocin antagonist. These findings suggest that although functional oxytocin receptors did not appear to be essential for the initiation of labor, oxytocin antagonists may still be effective in the prevention of premature contractions. We also examined the effects of a higher dose of naproxen sodium, 5.0 mg/day, on gap junction formation. At this dose, naproxen sodium suppressed both oxytocin receptor and gap junction formation, prolonged gestation, and delayed parturition by 24 h or longer. Prostaglandin appears to be an important regulator or mediator of oxytocin receptor and gap junction formation and plays a critical role in the initiation of labor.Key words: oxytocin, prostaglandin, oxytocin receptor, gap junction, labor.