The localization of pheromone receptors in the rat vomeronasal epithelium was examined by light- and electron-microscopic immunocytochemical analysis, using affinity-purified polyclonal antibodies. The antibodies were raised against a synthetic oligopeptide corresponding to a partial sequence of the rat putative pheromone receptor (VN6). Positive immunoreactivity was observed on the luminal surface of the sensory epithelium, and was abolished when an excess of the antigen peptide was added to the primary reaction solution. On electron microscopy, the immunoreactivity for the VN6 peptide was localized at the dendritic knobs and microvilli of receptor cells, but not in those of the supporting cells. These results show the first evidence of cellular localization of putative pheromone receptors in rat vomeronasal receptor cells.