Four normally cycling women received an iv injection of 20 mg Premarin (conjugated estrogens equivalent to 20 mg estrone sulfate) on the seventh day of two consecutive cycles; the second experiment was performed under sulpiride-induced hyperprolactinemia (mean PRL level = 906 microU MRC standard 71/222 per ml; significantly 7.8 times greater than mean control level of 115 microU MRC standard 71/222 per ml, P less than 0.001). In comparison to the control experiment, sulpiride-induced hyperprolactinemia prevented the occurrence of any gonadotropin peak within the 84 h of estrogen administration; the negative feedback effect of estrogen on gonadotropin secretion was maintained and was even potentiated. These alterations of feedback mechanisms of estrogen were considered to be related to hyperprolactinemia itself rather than to sulpiride. Five other normally cycling women received iv injections of 100 microgram LRH on the 22nd day of a cycle under sulpiride-induced hyperprolactinemia since the onset of menstruation. Their mean LH response was somewhat greater (although not statistically significant) and their mean FSH response was considerably greater (P less than 0.001 at all times) than those of a control group of 10 women tested in their luteal phase. The results of these LRH tests under sulpiride-induced hyperprolactinemia give some support to the concept that hyperprolactinemia interferes at the hypothalamic or higher level with cyclic release of indogenous LRH.