Administration of an intravenous bolus (1.0 mg) or a constant rate infusion (250 microgram/h for 4 h) of 2-hydroxyestradiol (20H-E2) to hypogonadal women, resulted in no discernible alteration in the circulating levels of LH and FSH. In contrast, progressive increments in the release of prolactin were unequivocally detected at 4.5 h after receipt of the bolus and 2.5 h following the initiation of the infusion.