Ballon-tipped fluid-filled catheters were used to study the effects of prostaglandin (PG) E1, E2, F1α, and F2α on the muscular activity of the oviductal isthmus in unanesthetized unmated New Zealand rabbits. Prostaglandins of the E series suppressed spontaneous activity and often completely abolished oviductal muscle activity. PGF1α and PGF2α effected an increase in muscular activity of the oviduct, which was characterized by a maintained spasmodic contraction. The magnitude of response after the injection of prostaglandins was often minimal in animals having a low level of spontaneous activity, indicating that the effect of PGs may be related to other tubal muscle regulatory mechanisms. The duration of effect of E prostaglandins increased as the dose increased, whereas the length of the F prostaglandin-induced contraction was the same after the three dose levels used. The effects of E- and F-series prostaglandins on oviductal muscle activity were mutually antagonistic; F prostaglandins overcame the suppressive effect of E prostaglandins, and E prostaglandins abolished the increase in tubal activity caused by F prostaglandins. These antagonistic effects were dependent on the time interval between the administration of the prostaglandins. Also, PGE1 and E2 abolished spontaneous spasmodic contractions. The subcutaneous (sc) administration of PGE1 suppressed spontaneous oviductal activity for 5.4 h, whereas the suppressive effect of PGE2 after sc administration lasted only 1.4 h. The effects of similarly administered PGF1α and F2α lasted 3.0 and 3.4 h, respectively. In view of their opposite and antagonistic effects on oviductal muscle activity, the prostaglandins may be involved in the passage of embryos through the oviducts. The F prostaglandins may act to retain the embryos in the oviduct by virtue of their occlusive action, and the E prostaglandins may allow the embryos to pass into the uterus by abolishing tubal occlusion.