AbstractThe growth and differentiation of olfactory sensory neurons are regulated tightly. We had shown previously, by immunohistochemistry, that transforming growth factor‐α (TGF‐α) and epidermal growth factor (EGF) receptor are present in the olfactory epithelium of untreated adult rats and that TGF‐α is a potent mitogen of olfactory epitheliumin vitro. Expression of EGF receptor and TGF‐α was detected primarily in horizontal basal cells and supporting cells but rarely in globose basal cells, which suggested that EGF receptor is not a likely candidate for the mitotic regulator of sensory neurons. In order to expand the search for candidate regulators, we have now examined other members of the EGF family of receptors and ligands. By utilizing reverse transcriptase‐polymerase chain reaction (RT‐PCR) methodology, we have detected the messenger RNA encoding the protein of theneugene (p185neu) and Neu differentiation factor (NDF) isoforms in the olfactory mucosa. Immunohistochemical localization of p185neuand NDF indicates expression of these proteins in the olfactory epithelium of adult rats in regions where globose basal cells and immature sensory neurons are found, as well as in the ensheathing cells of the olfactory nerve. The presence ofneuand NDF transcripts in the olfactory tissue and the localization of their encoded polypeptides to proliferative regions of the epithelium suggest involvement of these gene products in the regulated proliferation/differentiation of the sensory neurons.