A mixed epithelial tumour in the liver of a 24-month-old male Wistar rat from a 30-month inhalation study is described. The rat, which was in a group exposed to low concentrations of diesel exhaust, was euthanized because of emaciation, forced respiration and abnormal gait. macroscopic examination of the enlarged liver revealed multiple partly confluent beige-red nodules up to 1.5 cm in diameter. Small nodules up to 7 mm in diameter were seen in the spleen. Histologically, the tumour nodules in the liver consisted of hepatocellular and cholangiocellular components. The hepatocellular component consisted of moderately differentiated polygonal to round hepatocytes about twice as large as normal hepatocytes and having hyperchromatic, Centrally located nuclei with prominent nucleoli and eosinophilic cytoplasm. Foci of haematopoiesis and focal necroses were prominent. The cholangiocellular component was moderately differentiated and consisted of tubular structures lined by low cuboidal to cylindrical cells showing cytoplasmic basophilia and small dark nuclei without prominent nucleoli. The histological features of the nodules in the spleen corresponded to those of the primary tumour in the liver. Based on these criteria, the tumour nodules were diagnosed as hepatocholangiocellular carcinoma. The immunohistological examination confirmed the diagnosis, i.e. immunostaining for cytokeratins was positive for eight and 18 (hepatocellular carcinoma) and for seven and 19 (cholangiocellular carcinoma) as well as for vimentin (dense fibrous stroma). This tumour is considered to be spontaneous because of its single occurrence.