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The agonistic behaviors of adult male golden hamsters (N = 108 dyads) were examined 5 min after stereotaxic microinjection of adrenal and gonadal steroids into the anterior hypothalamus. Flank marks, attacks, bites, and retreats were scored over a 15 min test period during which steroid-injected animals were paired in a neutral arena with vehicle-injected conspecifics. Animals microinjected with either 10(-6) M cortisol or 10(-6) M beta-estradiol displayed significantly (p less than 0.05) higher levels of flank marking than other steroid-treated animals. Animals microinjected with 10(-6) M cortisol displayed significantly higher levels of aggression than their opponents. In contrast, the behavior of the vehicle-injected animals paired with 10(-6) M cortisol-treated opponents was characterized by submissive responding. This profile of the 10(-6) M cortisol treatment, i.e., promoting aggression in a steroid-treated animal while eliciting submission from its vehicle-treated opponent, was not observed in pairs in which steroid-injected animals were treated with equimolar concentrations of testosterone, dihydrotestosterone, progesterone, beta-estradiol, or deoxycorticosterone. These findings suggest steroids exert immediate effects on agonistic responding in the anterior hypothalamus of male hamsters. The immediate action(s) of cortisol appear to include facilitating aggression and flank marking, while the immediate action(s) of beta-estradiol appears to be confined to the communicative aspect of agonistic responding in this species.