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The ability of 20 alpha-hydroxy-4-pregnen-3-one (20 alpha-dihydroprogesterone, 20 alpha-DHP) and its 5 alpha-reduced metabolites, 20 alpha-hydroxy-5 alpha-pregnan-3-one and 5 alpha-pregnane-3 alpha,20 alpha-diol, to facilitate ovulation was examined in immature female rats which had been treated on day 22 of age with a non-ovulatory dose (12 i.u.) of pregnant mare serum gonadotrophin. An increased incidence of ovulation occurred in rats treated on the morning of day 24 with 20 alpha-DHP. However, a dose of 20 alpha-DHP three to four times that of progesterone was required to induce ovulation in all animals. In contrast, neither 20 alpha-hydroxy-5 alpha-pregnan-3-one (at doses ranging from 0.5 to 5.0 mg) nor 5 alpha-pregnane-3 alpha,20 alpha-diol (tested at 3.0 or 4.0 mg) facilitated ovulation. Therefore both 20 alpha-DHP and progesterone, the two major progestins of the rat oestrous cycle, have the ability to facilitate ovulation. It is concluded that the ability of 20 alpha-DHP to facilitate ovulation does not appear to be by way of conversion to its 5 alpha-reduced metabolites.

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