A progesterone-dependent step in ovulation induced by human chorionic gonadotrophin in immature rats primed with pregnant mare serum gonadotrophin.

In immature rats primed with pregnant mare serum gonadotrophin, antiserum to progesterone could prevent or reduce ovulation in response to injected human chorionic gonadotrophin (HCG). To be effective, antiserum treatment had to be within 6 h of gonadotrophin treatment; antiserum given 9 h after HCG was ineffective. Progesterone restored the antiserum blocked ovulation completely or incompletely when administered intravenously within 6 h of treatment with HCG. The first 6 h was shown to be a progesterone-dependent step in the ovulatory process in this experimental system.