The vomeronasal system of mammals is chemoarchitecturally dichotomous. Two populations of receptor cells have been identified in the vomeronasal sensory epithelium based on the family of receptor proteins they express on their membranes. These two receptor cell populations express different G-proteins: the more basal population expresses G oโฃ and the more apical population expresses G iโฃ2 . The G oโฃ -expressing receptor cells project their axons to the posterior accessory olfactory bulb (AOB) whereas the G iโฃ2 -expressing cells project their axons to the anterior AOB. In all mammals studied to date, the anterior AOB is G iโฃ2 -positive and the posterior AOB is G oโฃ -positive. These two parts of the AOB are also chemoarchitecturally heterogenous with respect to their carbohydrate content as revealed both with lectin binding and immunoreactivity to monoclonal antibodies raised against carbohydrate moieties. However, species differences have been observed with respect to lectin binding, as with NADPH-diaphorase reactions and OMP immunoreactivity. Recent studies indicate that there are physiological and behavioral correlates to the dichotomy within the vomeronasal system.