In this study the effect of recombinant human follicle stimulating hormone (rFSH) on oestradiol production by human granulosa-lutein cells was examined in long-term culture, in the presence or absence of androgens. Cells were harvested at the time of follicular aspiration after ovarian hyperstimulation for in-vitro fertilization and cultured for 9 days. Granulosa cells were capable of secreting oestradiol spontaneously even without androgen and gonadotrophin support. Basal oestradiol secretion was relatively high and variable (421.3 +/- 159.3 pg/ml, mean +/- SEM, n = 13) on the first day and decreased gradually to 16.7 +/- 3.1 pg/ml on day 9. Addition of androgens (testosterone or androstenedione) to the incubation medium enhanced dose-dependently basal oestrogen production on days 5, 7 and 9. The androgen/oestrogen conversion rate remained constantly high during the culture, even without rFSH. After pre-incubation for 3 days, addition of rFSH resulted in a dose- and time-dependent increase in granulosa-lutein cell oestrogen production in the absence of exogenous androgens. Testosterone supplementation caused considerably higher basal oestradiol concentrations, however rFSH failed to further stimulate the oestrogen release from granulosa-lutein cells, suggesting that these cells cultured in vitro possess high aromatase activity even without rFSH support.