Using an antibody raised against an odor receptor protein, we investigated immunohistochemically the spatial distribution in the embryonic and adult rat olfactory epithelium of the olfactory receptor neurons that express the odor receptor protein. In adults, the immunoreactive olfactory receptor neurons were intermingled with immuno-negative receptor neurons, but were mostly restricted within a circumferential zone located in the lateral part of the epithelium. The immunoreactive olfactory receptor neurons were observed as early as embryonic day 14, with a strong tendency to localize in the lateral part of the epithelium. These results indicate that both selection of the odor receptor protein by individual olfactory receptor neurons and zonal segregation of the odor receptor protein expression occur early in embryonic development of the olfactory system.