To examine the manner in which the FSH surge of one oestrous cycle recruits follicles for ovulation in the subsequent cycle, porcine follicular fluid (PFF) was used to alter the pattern of endogenous FSH secretion during the periovulatory period. OVaries of animals killed at oestrus or metoestrus were examined histologically for the presence of large follicles (greater than 400 micrometers in diameter) after treatment. Large follicles were absent in ovaries of PFF-treated animals at oestrus, while control rats had an average of 2.7 large follicles per ovary. By metoestrus, however, ovaries of rats treated with PFF contained several large, healthy follicles. Only when PFF treatment was continued throughout the evening of oestrus was the appearance of large follicles prevented at metoestrus. Our results suggest that the prolonged oestrus portion of the FSH surge, rather than the pro-oestrous portion, is responsible for follicular recruitment during the normal oestrous cycle in the rat. They also indicate that compensatory follicular development occurs in response to the FSH rebound which has been shown to follow FSH suppression.