The increased serum concentration of follicle-stimulating hormone (FSH; follitropin) between proestrus and estrus in the rat has been hypothesized to recruit the follicles destined to ovulate in the next cycle. Injection of porcine follicular fluid (PFF) late in proestrus suppresses the secondary FSH surge; injection early in proestrus suppresses the primary FSH surge without affecting the secondary FSH surge. Thus, it is possible to use PFF to test the FSH/follicular recruitment hypothesis and to distinguish between the contributions of the primary and secondary FSH surges to this recruitment. The normal recruitment of follicles occurs in the diameter range 350-499 microns between the day of proestrus and the day of estrus. When the secondary FSH surge was suppressed by injection of PFF late in proestrus, PFF, but not porcine serum (PS), blocked follicular recruitmenet into size groups of 350-499 microns on the morning of estrus. The number of ova ovulated did not differ between PFF- and PS-treated animals. When we suppressed only the primary FSH surge, by injecting PFF early in proestrus, there were no differences between PFF- and PS-treated animals in the number of ova ovulated, follicle size distribution, or hormones. In the last experiment, the secondary FSH surge was blocked with PFF but was replaced with exogenous ovine FSH which caused a dose-related increase in follicular recruitment, substantiating the interpretation that the follicular fluid suppressed recruitment by suppressing FSH secretion. Thus, in mammals with short reproductive cycles, the gonadotropin surges provide a "fail-safe" mechanism whereby luteinizing hormone triggers ovulation, thus ending one cycle, and the secondary increase in FSH levels recruits follicles for the next cycle.