The extrapulmonary airways of 8 male beagle dogs were examined histopathologically after continuous exposure to a sulphite aerosol at a concentration of 1 mg/m3 for 290 days. Hyperplastic foci were observed in the respiratory region of the posterior nasal cavity. The changes were characterized by a thickened epithelial layer resulting from epithelial proliferation, by a loss of secretory material and by moderate mononuclear cell infiltration. In the larynx an increased number of non-ciliated cells, and slight mononuclear cell infiltration were found. Tracheal changes were limited to an increased number of non-ciliated cells in the membranous portion. TEM (transmission electron microscopy) showed the changes in the trachea to be caused by disorder in the development of ciliated cells. These findings suggest that sulphite aerosols have adverse effects on the extrapulmonary airways of beagle dogs.