PIP: The response of serum LH, FSH, and 17beta-estradiol (17b-E) to stimulation with D-Tryp6-LH-releasing hormone (LH-RH analog) was measured in 34 normal women. The analog was administered either by vein, muscle, or continuous iv infusion in doses from 1 to 50 mcg during different phases of the menstrual cycle. The increase in LH and FSH to 10 mcg analog was significant (p .05) in all phases of the menstrual cycle by 60 minutes postadministration and lasted for at least 8 hours. 17b-E increased significantly by 8 hours. LH-RH analog was 40 and 21 times more potent than LH-RH in the stimulation of LH and FSH, respectively, during the follicular phase. In the ovulatory phase, LH and FSH responses differed kinetically from their responses in the other phases, and gonadotropin release was both faster and greater. The gonadotropin responses of 3 men when compared with those of the female subjects were almost always smaller (p .05). Because of the greater potency of this LH-RH analog, it may be useful in the treatment of infertility.