Testosterone or its metabolite, estrogen, regulates aggression in males of many mammalian species. Because plasma testosterone levels are typically positively correlated with both aggression and reproduction, aggression is expected to be higher when males are in reproductive condition. However, in some photoperiodic species such as Siberian hamsters (Phodopus sungorus), males are significantly more aggressive in short day lengths when the testes are regressed and circulating testosterone concentrations are reduced. These results led to the formation of the hypothesis that aggression is modulated independently of circulating steroids in Siberian hamsters. Thus, recent studies have been designed to characterize the role of other neuroendocrine factors in modulating aggression. However, aggression may be mediated by testosterone or estrogen despite basal concentrations of these steroids by increasing sensitivity to steroids in specific brain regions. Consistent with this hypothesis, we found that males housed under short days have increased expression of estrogen receptor alpha in the bed nucleus of the stria terminalis, medial amygdala, and central amygdala. Neural activation in response to an aggressive encounter was also examined across photoperiod.