The behavioral responses of the female golden hamster to a sexually experienced male were analyzed as a function of the stage of the female's estrous cycle. Exogenous estradiol or estradiol followed by progesterone was given to ovariectomized females to determine the role of these hormones in the regulation of cyclic changes in the female's response to the male. Females were paired on a daily basis with sexually active males for 10 min, and behavioral interactions were recorded. Significantly more fighting occurred during early diestrus than later in the cycle. During the 24 hr preceding sexual receptivity, fighting was infrequent. BY 8 HR PRIOR TO THE ONSET OF SEXUAL RECEPTIVITY, 86% OF THE POPULATION EXHIBITED A BEHAVIOR RESEMBLING THE ONSET OF LORDOSIS WITHOUT IMMOBILIZATION (PRELORDOSIS). Following ovariectomy fighting was at a high level (80%--90%). Estradiol replacement, over a 28-day test period, resulted in a significant decrease in aggression and a significant increase in the display of the prelordotic response without inducing lordosis. Initially, the treatment with progesterone following 7 days of estradiol treatment caused the female to display lordosis. After 24 hr a significant increase in aggression, which continued as long as progesterone was present, was observed. After removal of progesterone a significant decrease in aggression occurred. Thus, estradiol causes the female to become tolerant of the male's approach, the female exhibiting prelordosis in response to the male's investigation. Estradiol and progesterone are necessary for normal sexual receptivity; however, after 24 hr, estrogen-progesterone-treated females become agonistic to the male.