Daily treatment (5 days) with estradiol resulted in increased levels (p less than 0.05) of proceptive behavior in ovariectomized as compared to control mares (N = 8 per treatment) within 4 hr of injection and for the 4 subsequent days. Ejaculations occurred more often (p less than 0.05) in estrogen-treated mares on days 2-5, but the number of precopulatory investigations by the stallions was not altered. Progesterone treatment resulted in an absence of sexual behavior except in one mare on Day 1. Control mares exhibited varying levels of sexual interest. The concurrent administration of estradiol and progesterone produced a biphasic effect on proximity-related behaviors. Proximity behaviors were initially (Day 1) greater and subsequently less in the group treated with both hormones than in the group treated with estradiol alone. Injections of free estradiol resulted in a shorter latency to effect for two measures of proceptivity than did injections of estradiol benzoate. A dose response test for progesterone showed no effect with 0, 1, or 10 mg, but 100 mg was inhibitory (p less than 0.05). These results demonstrated that within 4 hr estradiol stimulated, while progesterone inhibited estrous behavior in ovariectomized mares and that concurrent administration of estradiol and progesterone produced a biphasic effect, first enhancing, then suppressing some aspects of the estrous response. The relatively short latency to action of all treatments and the biphasic effect of concurrent estradiol and progesterone may be attributable to low level, endogenous hormones.