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When bovine follicular fluid (BFF) was given i.p. three times at intervals of 3 h from 17.00 to 23.00 h to dioestrous rats pretreated with 10 i.u. human chorionic gonadotrophin (HCG) at 17.00 h on the day of dioestrus (day 0), the selective surge of FSH at 02.00 h on day 1 was suppressed in a dose-dependent manner. Three i.p. injections of 0·5 ml BFF completely suppressed the FSH rise in plasma at 02.00 h on day 1, but the time of premature ovulation induced by HCG was not altered. In these animals treated with HCG and BFF, however, the selective surge of FSH occurred as a delayed surge from 05.00 to 23.00 h on day 1. After seven i.p. injections of 0·5 ml BFF (from 17.00 h on day 0 to 11.00 h on day 1) the delayed surge of FSH took place from 17.00 h on day 1 to 11.00 h on day 2, indicating that waning of BFF with a decrease in inhibin secretion by the ovaries may be responsible for the delay of the FSH surge.

The next spontaneous ovulation in rats treated with HCG and BFF occurred on day 5, a delay of ovulation of 1 day compared with animals given HCG on day 0 with no BFF. Initiation of follicular maturation or selection of growing follicles for the succeeding oestrous cycle appeared to be retarded by the delay of the FSH surge in HCG- and BFF-treated animals.

The pituitary content of FSH in animals given HCG and three i.p. injections of 0·5 ml BFF increased strikingly until 11.00 h on day 1, when the delayed FSH surge was already in progress. These results suggest that the ability of the pituitary gland to synthesize FSH is high during the period of ovulation.