The role of estrogen in modulation of agonistic (attack-chase) and proceptive (vaginal-marking) responses of the female hamster toward a male was examined in relation to time course of induction and duration of response. In addition, the sex-specific nature of the vaginal-marking response was examined. Cycling females showed a significantly higher frequency of attack-chase when endogenous estrogen levels were minimal in comparison to that at peak estrogen levels during the estrous cycle. Removal of estrogen (ovariectomy) resulted in a significant increase in attack-chase scores compared to high endogenous estrogen. Addition of exogenous estrogen (Silastic capsule) resulted in a significant decrease in attack-chase scores compared to ovariectomized animals. The inverse effect was found for vaginal marking with the lowest frequency of vaginal marking at low endogenous estrogen levels or following ovariectomy. During high endogenous estrogen levels or following treatment with exogenous estrogen vaginal-marking scores were maximal. Maximal induction of vaginal-marking responses and reduction of attack-chase were found between 6 and 48 hr of estrogen treatment. Both behaviors then returned to the prehormonal treatment level. Failure to detect effects beyond 48 hr was not due to repeated testing, as animals lacking hormone, or animals first tested at 96 hr of estrogen exposure, showed no effect of the treatment. Males gonadectomized as adults never showed vaginal marking, whereas males gonadectomized neonatally showed a low level of vaginal marking which was increased significantly following estrogen treatment. Thus both attack-chase and vaginal marking in the presence of a male are modulated by estrogen. Further, since vaginal marking is estrogen sensitive in the neonatally gonadectomized male, expression of vaginal marking is determined by the hormonal environment during the neonatal period. However, peripheral feedback may be important for frequency of occurrence.