The present study was performed to elucidate the factors which modulate uterine prostaglandin synthesis during the implantation period in rats, by employing delayed implantation model. Administration of estradiol sharply increased uterine cAMP concentration 4 hrs later during the delayed implantation process. Concentrations of uterine PGE and PGF2α were increased at 12 hrs after the estradiol treatment although an increase in PGF2α was not statistically significant. The concomitant treatment of indomethacin with estradiol significantly suppressed estradiol-induced PGE and PGF2α at 12 hrs, while uterine cAMP concentration was not suppressed. The treatment of dbcAMP without estradiol gradually increased uterine PGE and PGF2α showing the maximum 8 hrs later, suggesting that cAMP minics estradiol effect on uterine prostaglandin synthesis during the implantation process. Furthermore, the pretreatment of theophylline, phosphodiesterase inhibitor, induced significantly greater concentrations of uterine PGE and PGF2α, compared with estradiol-only treated group. These results suggest that estradiol stimulates uterine prostaglandin synthesis and this process may be mediated by an elevation of cAMP during the delayed implantation process in rats.