The objective of this study was to assess the effect of ovine follicular fluid (FF) treatment (with or without FSH replacement) during the late follicular phase on plasma concentrations of gonadotrophins and the development of the ovulatory follicle. Ovarian steroid secretion and expression of mRNA encoding inhibin alpha and beta A, beta B subunits, P450 aromatase and P450 17 alpha-hydroxylase were used as endpoints. After induction of luteolysis by injection of 100 micrograms cloprostenol on days 10-12, Scottish Blackface ewes were allocated to one of three groups: (1) control (n = 7): no further treatment; (2) FF (n = 9): subcutaneous injections of 3 ml steroid-free ovine follicular fluid at 9 h intervals, 18 and 27 h after cloprostenol injection; (3) FF + FSH (n = 8): injections of follicular fluid as above plus subcutaneous injections of 0.36 iu ovine FSH at 6 h intervals, 18, 24, and 30 h after cloprostenol injection. Jugular venous blood samples were obtained via indwelling cannulae at 6 h intervals from 0 to 36 h after cloprostenol injection, and at 10 min intervals from 12 to 18 h (control phase) and from 30 to 36 h after cloprostenol injection (treatment phase). At laparotomy, 36 h after cloprostenol injection, ovarian venous blood was collected and ovaries were removed and processed for in situ hybridization. Plasma concentrations of FSH, luteinizing hormone (LH) and oestradiol were determined by radioimmunoassay. Follicular fluid treatment resulted in a decrease (P < 0.001) in FSH concentrations associated with an acute decrease in ovarian steroid secretion (P < 0.01) and a specific depression in P450 aromatase, (P < 0.001), inhibin-activin beta B subunit (P < 0.05) and thecal LH receptor (P < 0.001) expression. Follicular fluid treatment had no effect on inhibin-activin alpha and beta A, subunit or P450 17 alpha-hydroxylase expression. FSH co-treatment with follicular fluid restored circulating FSH concentrations to normal values and reversed some of the effects of follicular fluid (androstenedione, testosterone and progesterone secretion, and inhibin beta B and thecal LH receptor expression) but not oestradiol secretion or P450 aromatase expression. It was concluded that the actions of follicular fluid are mediated via both central effects on pituitary FSH secretion and by direct ovarian effects on granulosa cell aromatase activity. The results indicate that follicular fluid contains a factor that inhibits aromatase activity of granulosa cells directly and may play a role in the selection of the dominant follicle.