Testosterone (T) and its androgenic and estrogenic metabolites modulate the ability of serotonin (5-HT)1A and 5-HT1B agonists to inhibit intermale aggressive behavior. This study tested whether the lateral septum (LS) and medial preoptic area (MPO), which are part of the neuroanatomical substrate for aggression and contain androgen, estrogen, 5-HT1A and 5-HT1B receptors, represent sites where these modulatory effects occur. Gonadectomized CF-1 male mice were given silastic implants containing diethylstilbestrol (DES, a synthetic estrogen) or dihydrotestosterone (DHT, a nonaromatizable androgen) and implanted bilaterally with guide cannula directed at the LS or MPO. They were microinjected with either CGS12066B, a 5-HT1B agonist (400 microM LS, 200 microM MPO); 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT1A agonist (10 microM LS, 5 microM MPO); or combined CGS + 8-OH-DPAT treatment and tested for aggression 15 min later. When microinjections were given in the LS, androgen-treated males exhibited significantly reduced attack behavior in response to CGS or to CGS + 8-OH-D PAT. The attack behavior of DES-treated males was not reduced by any of the treatments. In contrast, all agonist treatments decreased aggression when injected into the MPO in both hormone conditions. The findings demonstrate regional variation in the ability of androgens and estrogens to modulate 5-HT1A- and 5-HT1B-agonist mediated reductions in aggression.