We examined the effect of prostaglandin F2 alpha (PGF2 alpha) on phosphoinositide (PI)-hydrolysis in the outer layer of the human myometrium at the end of pregnancy. After pretreatment of myometrial explants with a cyclo-oxygenase inhibitor (indomethacin), the PGF2 alpha dose-response curve was shifted to the left and a maximal level of PGF2 alpha-induced IP accumulation was reached. In contrast, the well-known OT-induced IP accumulation was decreased in the presence of indomethacin, whereas potency was unchanged. Incubation of the myometrial explants with calcium-depleted medium, not only decreased the basal IP production but completely abolished the OT- and PGF2 alpha-induced IP accumulation. A L-type calcium channel inhibitor, verapamil 100 microM, significantly decreased PGF2 alpha-induced total inositol production whereas it had no effect on OT-response, suggesting that the first event of PGF2 alpha action is the activation of a calcium channel. These results point to marked differences in the mechanisms by which OT and PGF2 alpha exert their contractile effects in human myometrium at the end of pregnancy.