Five Japanese women with regular menstrual cycles (control group) and 5 sulpiride-induced hyperprolactinemic women (sulpiride group) were given a 100-microgram synthetic luteinizing hormone releasing factor (LRF) intravenously, on the 20th day of the menstrual cycle and on the 10th day of the next cycle. The mean responses of luteinizing hormone and follicle-stimulating hormone to LRF in the sulpiride group were higher than those in the controls. Despite enhancement of the gonadotropin levels, the mean response of estradiol was suppressed significantly in the sulpiride group in the luteal and follicular phase of the menstrual cycle. However, the mean response of progesterone was not statistically different, in either group, in the luteal phase. The results suggest that sulpiride-induced hyperprolactinemia exerts an inhibitory effect on steroidogenic activity in estradiol but not in progesterone in the ovary.