The basal levels of adenosine 3':5'-monophosphate (cyclic AMP) in the oviductal isthmus were lower during pseudopregnancy than during estrus. No differences were observed in the ampulla. Prostaglandin E2(PGE2) significantly increased cyclic AMP levels in the isthmus during pseudopregnancy but not during estrus. In contrast, in the ampulla, PGE2 did not alter cyclic AMP levels in either hormonal condition. Theophylline increased cyclic AMP levels in the isthmus and in the ampulla during pseudopregnancy. These results show that PGE2 causes a specific increase in cyclic AMP levels in the isthmus after ovulation.