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The present investigation was performed to elucidate the mechanism of the initiation of follicular maturation after inhibition of ovulation in rats treated with pentobarbitone sodium at 13.30 h and progesterone at 14.00 h on the day of pro-oestrus (day 0 denotes the day of these treatments). Ovulation was completely inhibited and the next spontaneous ovulation occurred on day 5, the expected day of the next oestrus. Follicular responsiveness to injection of human chorionic gonadotrophin (hCG) indicated that preovulatory follicles at the time of treatment with pentobarbitone and progesterone regressed by 05.00 h on day 2. Maturation of a new set of follicles began from 17.00 h on day 2 and all rats were induced to ovulate by hCG injection by 17.00 h on day 3, the number of oocytes ovulated being comparable to normal ovulation.

In the animals receiving pentobarbitone sodium and progesterone treatment, two selective rises in plasma FSH, which had peak levels at 05.00 h on day 1 and 11.00 h on day 2, were observed without a rise in LH. Preovulatory surges of FSH and LH occurred on the afternoon of day 4.

These results suggest that the second rise in FSH was induced by regression of Graafian follicles present at the time of treatment with pentobarbitone sodium and progesterone and that this surge of FSH was responsible for initiation of maturation of a new set of follicles destined to ovulate in the subsequent cycle. The mechanism of induction and the role of the first rise of FSH from the night of day 0 to the morning of day 1 cannot be explained at present.

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