Prostaglandins (PGs) are known to exert profound physiological and pharmacological effects on the female reproductive system in women and in many other species (Weeks, 1972; Labhsetwar, 1974). Indomethacin and aspirin, inhibitors of prostaglandin biosynthesis, block PMSG-induced ovulation in immature (Armstrong & Grinwich, 1972; Orczyk & Behrman, 1972) and adult female rats (Tsafriri, Lindner, Zor & Lamprecht, 1972). In the rabbit, ovulation induced by HCG, LH or coitus was blocked by indomethacin (O'Grady, Caldwell, Auletta & Speroff, 1972). In mice, ovulation was blocked with indomethacin and the effect was reversed by injecting prostaglandins E2 or F2α (Saksena, Lau & Shaikh, 1974). All these studies provide indirect evidence that PGs are involved in the process of ovulation. A more direct approach, namely the specific binding of endogenous PGs by anti-PG sera raised in rabbits, was used in the present study. Attempts were also made to reverse the antiovulatory effect of