Immunohistochemical localization of protein gene product 9.5 (PGP 9.5) was examined in the primary olfactory system of the rat. Receptor cells were intensely immunopositive in the olfactory epithelium, vomeronasal organ and septal olfactory organ of Masara. The mitral cells and some of the short-axon cells of the main olfactory bulb (MOB) were also intensely immunopositive, while the tufted cells of the MOB and the mitral/tufted cells of the accessory olfactory bulb varied in immunoreactivity.