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In immature female rats injected with PMSG at 30 days of age (day 30), ovulation occurs between the hours of 02.00 and 03.00 on day 33. If progesterone is injected at 10.00 h on day 32, the onset of ovulation is advanced by 1–2 h. In rats that were not given progesterone, ovulation was blocked by phenobarbitone sodium administered on day 32 before 13.50 h. However, pretreatment with progesterone at 10.00 h caused ovulation to occur in spite of phenobarbitone treatment at 13.50 h. An early release of ovulatory gonadotrophin from the anterior pituitary gland cannot completely account for progesterone's capacity to reverse the blockade of ovulation by phenobarbitone, because when phenobarbitone treatment was advanced by 2–4 h, ovulation still occurred in most progesterone-treated rats.