Seventy-eight follicles and their follicular fluid were aspirated from 46 women undergoing in vitro fertilization (IVF) procedures after stimulation of the ovaries with a low-dose human menopausal gonadotropin/human chorionic gonadotropin stimulation regimen. The concentrations of estradiol (E2), progesterone (P), testosterone (T), and prolactin (PRL) were measured in follicular fluid and related to the maturation of the oocyte-corona-cumulus complex (OCCC) and the fertilization of oocytes. Follicles containing mature oocytes had significantly higher follicular fluid E2 and P levels than follicles with intermediate and immature oocytes. A constant decrease in PRL and T values with advancing follicular maturation was observed. Similar results were obtained when the fertilizing ability of the oocytes was examined. The gradual decline in follicular fluid PRL and T levels during follicular development was connected with increasing E2 and P biosynthesis and therefore seems to be an important precondition for normal follicular and oocyte maturation.