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It is well known that ovulation is brought about by a surge of luteinizing hormone (LH). Ovarian steroidogenesis induced by LH has been implicated in the ovulatory process by the observation that ovulation is prevented by various inhibitors of steroidogenesis in immature rats primed with pregnant mare serum gonadotrophin (PMSG) (Lipner & Greep, 1971). Subsequently doubt was cast on this concept (Bullock & Kappauf, 1973), but recently it has been reported that a progesterone-dependent step exists in the normal ovulatory process in cyclic rats (Takahashi, Ford, Yoshinaga & Greep, 1974). However, the involvement of oestrogen in follicular rupture was not excluded in any of these experiments. In an attempt to obtain direct evidence of whether either steroid and, if so which, is essential for follicular rupture, the effects of neutralization of preovulatory progesterone or oestrogen with antisera to these steroids on induced ovulation was studied.Immature female Wistar rats received 5 i.u. PMSG subcutaneously on the morning of day 22, followed by injection into the tail vein of 10 i.u. human chorionic gonadotrophin (HCG) dissolved in 0-1 ml physiological saline 56 h later on the afternoon of day 24. Both oviducts were removed from the animals killed between 17 and 18 h after HCG injection, and tubai ova were counted under a dissecting microscope. Presence of even a single ovum in the oviduct was always accompanied by swelling of the ampulla. In questionable cases, where neither ovum nor swelling was recognized, the absence of intra-uterine ova was confirmed by flushing out the uterine contents. Various doses of rabbit antisera to progesterone (anti-progesterone) or to oestrone (anti-oestrone) were mixed with HCG solution and introduced into the rats intra¬ venously. Control animals received the same amount of normal rabbit serum (NRS) instead of the antisera. Anti-progesterone was raised against progesterone-3-carboxymethyloxime conju¬ gated with bovine serum albumin (BSA). A test of its maximum neutralizing capacity showed that 0-1 ml of a 103-fold diluted solution of the original antiserum bound 58-6% of 578 pg progesterone. Cross-reactivity with oestrone or oestradiol was below 0-06%. Anti-oestrone was raised against oestrorie-17-carboxymethyloxime conjugated to BSA, and had the following neutralizing capacity: 0-1 ml of a 5 103-fold diluted solution of original antiserum bound 59-7% of 442 pg oestrone. The antiserum showed 61-5% cross-reactivity with oestradiol, but below 0-04% with progesterone. In another series of experiments, the effects of replacement of progesterone or oestradiol after anti-progesterone administration on restoration of ovulation was examined. Thirty micrograms progesterone or 3 pg oestradiol dissolved in 0-5 ml NRS (Bischoff & Pilhorn, 1948) was introduced via the tail vein 1 h after HCG injection to the animals which had been given the highest dose of anti-progesterone (0-8 ml) simultaneously with HCG.As shown in Table 1, ovulation occurred in 100% of the rats which had received 0-1 or 0-2 ...

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