Immuno-electron microscopy was used to examine olfactory marker protein (OMP) ultrastructural localization in the supranuclear regions of the rat olfactory epithelium (OE) and vomeronasal organ (VNO) neuroepithelium. In the OE, OMP immuno-reaction product was observed within the cytoplasm of olfactory chemoreceptor cell dendrites, vesicles and cilia. Reaction product was absent from olfactory microvillar cells and their unique microvillar projections. In the neuroepithelium of the VNO, immuno-reaction product was seen within the dendrites of the chemoreceptor cells, and in the bases of their attached microvilli; the remaining distal portions of these microvilli were unlabeled. These results demonstrate a difference in the distribution of OMP immunoreactivity over the surfaces of the rat OE and VNO. They also show that OMP immunoreactivity does not exist in the rat olfactory microvillar cells.