We review studies conducted in mouse and ferret that have specified roles of both the main and the accessory olfactory nervous systems in the detection and processing of body odorants (e.g., urinary pheromones, extraorbital lacrimal gland secretions, major histocompatibility complex peptide ligands, and anal scent gland secretions) that play an essential role in sex discrimination and attraction between males and females leading to mate choice and successful reproduction. We also review literature that compares the forebrain processing of inputs from the two olfactory systems in the two sexes that underlies heterosexual partner preferences. Finally, we review experiments that raise the possibility that body odorants detected by the main olfactory system contribute to mate recognition in humans.