Regularly cycling rhesus monkeys received, from days 1 to 6 of the menstrual cycle (1) the potent luteinizing hormone-releasing hormone (LH-RH) agonist D-Trp6-LH-RH, 20 micrograms/day; (2) the potent LH-RH antagonist [N-Ac-D-Trp1,3,D-p-Cl-Phe2,D-Phe6-D-Ala10]-LH-RH, 1 mg/day; or (3) vehicle. Whereas control animals showed normal menstrual cycles, as determined by dates of ovulation, length of luteal phases, and hormonal profiles, animals treated with either analog of LH-RH exhibited disruption of the cycles. In animals from both groups, delayed ovulation was observed (LH-RH agonist, 22, 23, 17, 19, and 20 days of the cycle; LH-RH antagonist, 22, 22, 24, and 10 days of the cycle, and one animal remained anovulatory for 65 days). Monkeys treated with either LH-RH analog showed normal luteal phase lengths (15, 15, 16, 14, and 15 days, and 15, 16, 16, and 13 days, respectively) and serum progesterone concentrations. The results of this study suggest that, in the rhesus monkey, the administration of LH-RH analogs during the early follicular phase induces a temporary cessation of folliculogenesis demonstrated by a delay of ovulation, with subsequent normal luteal function.