The plasma concentrations of immunoreactive LRH, LH, and FSH were determined by RIA every 6 h until 72 h after iv administration of conjugated estrogens during the midfollicular phase. The percentage change in LH from the preinjection level showed a biphasic pattern after the injection of conjugated estrogens, i.e. significant suppression (-70%) from 6-42 h after the injection, followed by a rebound increase with a peak (+150%) at 56 h. Plasma FSH after the injection also showed a biphasic pattern. The plasma immunoreactive LRH levels were unchanged until 32 h after the injection, but then increased significantly (P less than 0.02) to 160% of the preinjection level at 42 h and then decreased rapidly. These data indicate that 1) estrogen administration results in increases in plasma immunoreactive LRH and LH, and the peak of plasma LRH precedes that of gonadotropin; and 2) the negative feedback effect of estrogen on gonadotropin secretion may not be mediated through LRH.