A case of a mucoepidermoid carcinoma, conventionally classified as an adenosquamous carcinoma, is described. The tumour bearing rat was exposed to a mixture of a pyrolized pitch condensate rich in polycyclic aromatic hydrocarbons and carbon black particles by inhalation for 10 months. The neoplasm was examined by conventional histopathologic procedures and by immunohistochemical detection of intermediate filaments. Morphologically, the tumour consisted of two components. The centre of the neoplasm was predominantly of adenocarcinomatous tissue and this was surrounded by keratinized squamous epithelium. The predominantly adenocarcinomatous component had a characteristic structural pattern consisting of one or a few layers of squamous epithelium covered by a continuous layer of mature goblet cells. The flattened cells were recognizable as squamous cells on the light microscopic level only after immunohistochemical staining with cytokeratin antibodies. Goblet cells and extracellular mucin were intensely positive for the PAS-reagent. This mucoepidermoid carcinoma in the rat was morphologically similar to those described in man. It is still unclear whether pulmonary mucoepidermoid carcinomas of humans originate from the bronchial epithelium or bronchial glands. It is most probable that the mucoepidermoid carcinoma of a rat described in this communication occurred by metaplasia in a carcinoma of bronchiolo-alveolar origin.