The relationship between a surge of FSH and the initiation of follicular maturation was examined using rats with a 4-day oestrous cycle. When antiserum against LH-releasing hormone (LHRH-AS) was given at 13.00 h on the day of pro-oestrus (day 0), surges of FSH and LH were blocked. Plasma FSH and LH were maintained at low basal levels without a surge release until the next spontaneous surge occurred on the afternoon of day 4, the predicted day of pro-oestrus. Follicular responsiveness to an injection of human chorionic gonadotrophin (hCG) indicated that preovulatory follicles, present at the time of treatment with LHRH-AS, were capable of ovulating on day 1 but had regressed by day 2. Subsequently, as shown by the ovulatory response to hCG, a new set of follicles had begun to mature by the morning of day 3 without a preceding surge of FSH. Changes in oestradiol-17 beta levels in the plasma throughout the oestrous cycle were the same in rats injected with LHRH-AS or non-immune control serum. The mature follicles in the rats treated with LHRH-AS thus retained the capacity to ovulate after losing their ability to secrete oestrogen. These results suggest that an FSH surge is not essential for initiation of follicular maturation and that basal levels of FSH may be enough to initiate follicular maturation in the absence of newly formed corpora lutea.