A new analog of luteinizing-hormone-releasing factor, [Des-Gly-NH210, Pro-ethylamide9]-LHRF, was tested in healthy women with regular menstrual cycles. This synthetic nonapeptide amide was injected intravenously in a dose of 200 µg into 15 normal female volunteers in the follicular, preovulatory, and luteal phases of the menstrual cycle, and the serum LH and FSH responses were compared with those to the same dose of the synthetic decapeptide LHRF in 15 controls. The new analog exhibited significantly higher activity than the synthetic decapeptide LHRF in the release of LH in the follicular (P<0.05), prcovulatory (P<0.01), and luteal phases (P<0.05). In contrast, no statistical significance in the magnitude of FSH response was demonstrated in any phase of the menstrual cycle. The serum LH and FSH response to the nonapeptide amide suggested a longer physiologic half-life of the new LHRF analog. The results are similar to a previous report in men with normal gonadal function, and a clinical usefulness for the new analog is expected.