Fluids were collected from 136 ovarian follicles of 35 women undergoing in vitro fertilization and embryo transfer (IVF-ET). Fifteen women (76 follicles) received oral contraceptive pills (OCs) prior to ovulation induction. All women received human menopausal gonadotropins (hMG) for ovulation induction and in all cases follicular aspiration was performed 32 to 34 hours after an injection of human chorionic gonadotropin (hCG). The concentrations of follicular-stimulating hormone (FSH), luteinizing hormone (LH), progesterone (P), and 17 beta-estradiol (E2) in the follicular fluids (FF) were measured by radioimmunoassay (RIA). FSH concentration in the FF of the OCs group (15 women, 76 follicles) was significantly lower (2.1 mIU/mL) as compared to the FSH (15.9 mIU/mL) in the FF of the control group (20 women, 60 follicles). The LH FF concentrations after hCG injection were similar in the two groups. The E2/P ratio in the OCs group (9.6) was significantly lower than the E2/P ratio in the control group (20.6). OCs given to patients before induction of ovulation with hMG results in lower E2/P ratios and lower FSH concentration in the FF.