Treatment of Damline ewes with i.v. injections of various doses (2, 5 or 10 ml) of bovine follicular fluid for 72 h after prostaglandin-induced luteal regression resulted in a significant decrease in plasma concentrations of FSH after a 1.5-2 h delay but did not affect LH. The half life of this decrease in plasma FSH levels (156 min) after injection of follicular fluid was similar to that for clearance (159 min) of ovine FSH after infusion. A significant rebound increase in plasma FSH levels occurred by 13 h after all follicular fluid injections, and the magnitude of this rebound was inversely related to the dose of follicular fluid injected. A significant delay in the onset of oestrus occurred only with 5 and 10 ml bovine follicular fluid. There was no significant effect on ovulation rate or subsequent corpus luteum function as measured by plasma concentrations of progesterone. Infusion of ovine FSH (50 micrograms/h for 48 h) during the period of follicular fluid treatment prevented the delay in onset of oestrus and resulted in a substantial (2-10-fold) increase in ovulation rate. Corpus luteum function in terms of progesterone secretion was also enhanced. These results show that (1) intermittent suppression of FSH during the preovulatory period in the ewe does not affect subsequent ovulation rate or corpus luteum function and (2) the delay in the onset of oestrus induced by bovine follicular fluid can be prevented by exogenous FSH.