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Treatment of immature rats with pregnant mare serum gonadotrophin followed by human chorionic gonadotrophin (HCG) caused an acute and temporary increase in concentrations of progesterone, testosterone and oestradiol in plasma with maximum levels 3 h after the administration of HCG.

Concurrent injection of indomethacin and HCG reduced, in a dose-dependent manner, the mean number of ova shed and this was accompanied by a dose-dependent decrease in concentrations of plasma progesterone and testosterone but not of oestradiol when they were measured 3 h after the injection of HCG. The minimum effective dose that blocked ovulation completely at 0 h abolished the acute increase of progesterone and testosterone, suggesting that prostaglandins act on ovulation by stimulating steroidogenesis at an early stage in the preovulatory process. The anti-ovulatory action of the minimum effective dose at 0 h became progressively less potent as the time between injection of HCG and administration of indomethacin was increased, although plasma concentrations of progesterone and testosterone measured at autopsy 18 h after treatment with HCG had not changed appreciably. When indomethacin was administered 10 h after HCG, the relationship between the dose of indomethacin and the mean number of ova differed from that observed when simultaneous injections of indomethacin and HCG were given, and the minimum effective dose that prevented ovulation was much higher than that at 0 h, suggesting that prostaglandins act differently on ovulation in the later stage of the preovulatory process.

It was concluded that prostaglandins may mediate the action of HCG on ovulation through two mechanisms which operate at different stages of the preovulatory process.