The anti-inflammatory steroid, dexamethasone, was demonstrated to have an antifertility effect in rabbits when administered systemically on Days 5-7 of pregnancy. The effects of this agent on prostaglandin (PG) concentrations and phospholipase activity in endometrium were determined. Using a highly specific and sensitive gas chromatography-mass spectometry (GC-MS) method, it was established that PGF2 alpha, PGE2 and 6-keto-PGF1 alpha concentrations are all significantly elevated in implant site endometrium relative to nonimplant regions on Day 7 postcoitus (p.c.). Although these levels were strikingly lowered in indomethacin-treated females, the administration of dexamethasone altered the balance between the various PGs but did not reduce tissue levels greatly. Determinations of PG levels in uterine venous plasma likewise demonstrated that neither synthesis nor release of PGs had been drastically curtailed by dexamethasone treatment. Endometrial phospholipase activity was determined on Days 6, 7 and 8 of pregnancy. Enzyme activity was elevated on Day 7 relative to the other days and implant site activity was significantly greater than nonimplant region activity on both Days 6 and 7. Treatment with either dexamethasone or indomethacin reduced the Day 7 activity to baseline levels and obliterated the differential between implant and nonimplant regions. The dexamethasone effect appears to be mediated by induction of a transferable inhibitory factor. Thus, while indomethacin and dexamethasone have similar effects on pregnancy when present during the implantation period, their respective mechanisms of action may be different. Even though dexamethasone inhibits endometrial phospholipase activity, this is not attended by inhibition of PG generation.