Feedback of oestrogen on gonadotrophin release was studied in five normally cycling normoprolactinaemic women who were rendered moderately hyperprolactinaemic by oral administration of sulpiride (150 mg daily) for 11 consecutive days. Five patients with peptic ulcer, who had been given 150 mg of sulpiride for 3-18 months and had chronic marked hyperprolactinaemia were also studied. Ethinyl oestradiol (200 micrograms) was administered by mouth daily for 4 consecutive days and serum concentration of prolactin, luteinizing hormone (LH), follicle stimulating hormone (FSH) and oestradiol were measured by a double-antibody radioimmunoassay for 11 days. Oral administration of ethinyl oestradiol caused an initial decrease followed by a rebound of LH with a peak in all the women tested. The serum FSH levels showed only the initial decrease, followed by a slight rebound. Serum oestradiol levels decreased during oral administration of ethinyl oestradiol in all the women. The results of the present investigation suggest that oestrogen feedback effect on LH release is maintained in sulpiride-induced hyperprolactinaemic women as well as in women with normal cycles.