Doses of 100 or 200 micrograms of a novel GnRH antagonist ([N-acetyl-D beta Na11-D-pCl-Phe2-D-Phe3-D-Arg6-Phe7-Arg8-D-Ala10]NH2 GnRH) (4 animals/dose) were administered on Days 10/11 of the luteal phase and induced a marked suppression of circulating bioactive LH and progesterone concentrations within 1 day of treatment (P less than 0.01). Thereafter, progesterone concentrations remained low or undetectable until after the next ovulation. Similar results were obtained when 200 micrograms antagonist were given on Days 5/6 of the luteal phase (N = 4). The interval from injection of antagonist (200 micrograms but not 100 micrograms) to ovulation (based on a rise in progesterone above 10 ng/ml) was significantly longer than that from prostaglandin-induced luteal regression to ovulation in control cycles (N = 4/treatment) (range, 13-15 days after antagonist vs 8-10 days after prostaglandin, P less than 0.01). This delay of 4-5 days was equivalent to the duration for which LH concentrations were significantly suppressed by 200 micrograms antagonist when administered to ovariectomized animals (N = 3). Corpus luteum function during the cycle after GnRH antagonist treatment appeared normal according to the pattern of circulating progesterone. These results show that corpus luteum function and preovulatory follicular development in the marmoset monkey are dependent on pituitary gonadotrophin secretion.