Previous studies in which prostaglandin (PG) production was inhibited for a limited time by the s.c. administration of indomethacin have suggested that PGs are involved in the initiation of decidualization as well as the growth and differentiation of decidual cells. To reduce PG production during decidualization, in the present study indomethacin was infused from Alzert osmotic minipumps into the uterine lumen of ovariectomized rats with uteri sensitized for decidualization. To determine the effect of route of indomethacin administration on decidualization, rats received a single s.c. injection of indomethacin or its vehicle, and unilateral intrauterine infusion of indomethacin or its vehicle, in a factorial experiment. The inhibitory effects on decidualization, as assessed 5 days later by uterine weights, were greatest when both treatments were combined. PGE and PGF concentrations 24 and 48 h after the insertion of the pumps were lower in the indomethacin-infused horns, suggesting that the indomethacin reduced uterine PG production. S.c. administered indomethacin reduced uterine PG concentrations at 24 h but not at 48 h. PGE2 and PGF2.alpha. alone or combined, infused with indomethacin into the uterine lumen of rats treated s.c. with indomethacin, overrode the inhibitory effects of indomethacin. The dose-response relationships between these PGs and decidualization did not differ. Apparently, PGs are required during the growth and differentiation of decidual cells from endometrial stromal cells.