A sensitive and specific double antibody radioimmunoassay for gonadotropin-releasing hormone (GnRH) has been developed for measurement in ethanol extracts of human plasma. Iodinated hormone was prepared with the use of the chloramine-T method, and antibodies were developed in rabbits over a six-month period with a GnRH synthetic copolymer immunogen. A Scatchard plot revealed at least three species of antibody. The assay can measure conservatively at the 5 pg. per milliliter level and shows no cross-reactivity with other available hypothalamic and pituitary hormones. The releasing hormone was quantitatively recovered from human plasma with immunologic identity to native hormone. Unextracted plasma could not be used because of nonspecific displacement. The measurement of GnRH in individuals receiving 100 mug of intravenous bolus infusions of the synthetic decapeptide show extremely elevated values with two half-lives: one of two to four minutes and another of 35 to 40 minutes. In our experiments, we have found measurable GnRH in patients with secondary amenorrhea and at the midcycle in normal women. In the normal cycling woman during the follicular and luteal phases, GnRH was undetectable. In postmenopausal women with extreme hypoestrogenism and markedly elevated luteinizing hormone values, GnRH was also undetectable. No bursts of GnRH could be detected in normal men when sampled every ten minutes over a two-hour period and every two hours throughout the day.