The rate of cell division in olfactory epithelium (OE) is upregulated by ablation of the olfactory bulb (Carr and Farbman, 1992), or downregulated by occlusion of a naris. We used an organ culture assay of fetal rat olfactory mucosa to study regulation of the mitotic rate. Addition of any one of three members of the epidermal growth factor (EGF) family-EGF, transforming growth factor-alpha (TGF-alpha), or amphiregulin (AR)-to a serum-free culture medium resulted in a two- to threefold increase in the number of dividing OE cells. TGF-alpha elicited a maximal response in a dose of 100-200 pM culture medium and was 2 orders of magnitude more potent than the other EGF family members. Addition of TGF-beta 1, TGF-beta 2, insulinlike growth factor-1 or platelet-derived growth factor to the culture medium had slightly less effect than EGF or AR, in about the same molar dose range; addition of nerve growth factor had virtually no net effect on cell division. Immunohistochemistry on adult rat OE showed that basal cells, supporting cells, and acinar cells of Bowman's glands were immunoreactive with antibody to TGF-alpha but not with antibody to EGF. Most growth factors upregulated division of both olfactory neuron progenitors and supporting cells. The data suggest that several growth factors, most prominently TGF-alpha 1, may participate in the mitotic regulation of OE.