In male hamsters, medial amygdala responds categorically to chemosensory stimuli, based on their species of origin or social relevance. Using immediate early gene (IEG) expression, we have demonstrated that the anterior medial amygdala (MeA) responds to both conspecific and heterospecific stimuli, but posterior medial amygdala (MeP) responds only to conspecific, socially-relevant stimuli, whether male or female. Heterospecific stimuli also activated the largely GABAergic intercalated nucleus (ICN) of the amygdala, adjacent to MeP, and suppressed GABA-receptor-ir cells in MeP, suggesting inhibition of MeP by ICN. A similar categorization occurs in mice (Samuelsen et al SfN-04). In hamster, it appears to be hard-wired and dependent on VNO input. Lesions of the main olfactory epithelium (OLFX) in sexually-naive males did not change the characteristic patterns of IEG activation in MeA/MeP elicited by conspecific and heterospecific stimuli, while removal of the VNO (VNX) eliminated categorization and most of th...