In two separate 24 month studies on the carcinogenic effect of single cadmium chloride injections in male Wistar (CRl:(WI)BR) rats, a total of 22% (129/597) of animals studied histologically were found to have chronic suppurative rhinitis caused by Aspergillus fumigatus. The diagnosis was based on characteristic conidial heads present in the sections, and positive methenamine-Grocott (GMS) staining of septate hyphae with dichotomous branching at angles of 45 degrees. Fungal hyphae balls, surrounded by a wall of neutrophilic granulocytes, were found in areas of the naso- and maxilloturbinates and occasionally caused complete blockage of the nasal passages. The underlying tissue showed an inflammatory response. In sections from 32 of the 129 cases (25% of the cases), epithelial necrosis and hemorrhage were indicative of fungal tissue invasion, but without dissemination to other organs. The infection rate was unaffected by the cadmium treatment or the location of rats in different cages. Positive antibody titers to Sendai and sialodacryoadenitis viruses suggested that transient inflammation of the upper respiratory tract rendered the mucosa susceptible to the fungal infection. The infection appeared to be sustained by growth around foreign bodies (hairs and plant material). Although focal squamous cell metaplasia of the respiratory epithelium with hyperplasia and hyperkeratosis occurred more frequently in rats with Aspergillus rhinitis, the incidence of tumors of the nasal cavities was not affected.