Charcoal-extracted porcine follicular fluid (PFF), when injected at 1200 h on proestrus, blocked the primary FSH surge seen at 1700 and at 1830 h without affecting the LH surge. In contrast, if the injection was withheld until 1330 h, the FSH surge occurred in the normal way at 1700 h but was suppressed at the 1830 h autopsy. The suppression of FSH by 0.5 ml PFF at 1200 h had abated by 0400 h of estrus. The blockade of the primary surge of FSH at 1700 and 1830 h did not prevent ovulation, nor did it prevent the expected rise in serum progesterone or fall of estradiol levels. There appears to be a latency of at least 3.5 h, when PFF is administered ip, before FSH can be suppressed. This latency occurs whether the injection is carried out in intact proestrous rats or in acutely or chronically ovariectomized females.