Abstract We report the first example of a complete inhibition of preovulatory gonadotropin secretion resulting from administration of a luteinizing hormone releasing hormone antagonist during a spontaneous menstrual cycle. The antagonist, [(<Glu-Pro) 1,D-Phe 2,D-Trp 3, 6]-LHRH, was administered to a rhesus monkey beginning on Day 9 of the menstrual cycle; ovulation did not occur and preovulatory peaks of LH and FSH were not observed despite elevations in serum estradiol-17β of sufficient strength and duration to elicit gonadotropin surges. Midcycle gonadotropin surges had already commenced in another monkey, however the antagonist did partially inhibit LH and FSH secretion although ovulation and luteinization were not prevented. Normal hormone secretion patterns and luteal function were observed in another monkey when the antagonist was given after the midcycle FSH and LH peaks had already occurred. These data emphasize the importance of beginning treatment with LHRH antagonists early in the follicular phase of the menstrual cycle.