We have reported that iv administration of conjugated estrogens results in no significant change in the plasma LH-RH level during the negative feedback phase of LH, suggesting that estrogen does not suppress LH by decreasing hypothalamic LH-RH. To determine the site of estrogen action during the negative feedback phase, we studied the pituitary response to a small amount of LH-RH after estrogen administration in normal cyclic women in the midfollicular phase. The pituitary responses to an iv bolus of 2.5μg of synthetic LH-RH were evaluated by measuring serum LH and FSH 2 h before and 8 h after administration of 20mg of conjugated estrogens (Premarin). The mean levels of serum LH and FSH were significantly (p<0.05) decreased 8 h after the injection. The peak responses of LH and FSH to LH-RH were also significantly (p<0.05) reduced after Premarin administration. These findings suggest that the negative feedback effect of estrogen on gonadotropin secretion is caused by its direct suppression on the pituitary response to LH-RH.