The immunohistochemical localization of proteins Gi1 (plus Gi3). Gi2 and Go was studied in the olfactory epithelium and the main olfactory bulb of rats, using purified antibodies to the respective α subunits and βγ subunits of these G proteins. In the olfactory epithelium, only a restricted population of olfactory cells was immunopositive for Gi2α, but others were not. The immunoreactivity for Gi1α/Gi3α was not observed. The olfactory epithelium was immunopositive for both Goα and βγ, but its apical surface was immunopositive only for βγ. In the main olfactory bulb, all layers were intensely immunopositive for Goα and βγ but weakly for Gi2α. In contrast to the negative or weak immunostainings in the olfactory nerve fiber layer and glomeruli, the molecular and the internal granular layers were intensely immunopositive for Gi1α/Gi3α. These findings suggest the functional difference among Gi1α/Gi3, Gi2 and Go in the signal transduction in the