An 11 years old Irish Setter bitch was euthanlzed and necropsied because of clinical findings such as severe purulent nasal discharge and formation of large tumor mass, 8 ${ imes}$8cm in size, in the abdominal cavity. A complete unilateral mastectomy had been carried out twice 14 and 22 months before necropsy. The surgically removed masess of the mammary glands had been diagnosed as malignant mixed tumor in each time. Grossly, tumor masses were observed in nasal cavity, infralumbar lymph node, lung, abdominal cavityn and brain. Microscopic findings of the surgically removed masses consisted of tumor epithelial cells, tumor hyaline cartilage-like structures and abundant connective tissues. The mass of the lymph node had similar microscopic features to those of the original malignant mixed tumor of the mammary glands. The tumor osseous tissue and osteoid were observed in the abdominal cavity, lung, and brain. Myoepithelial cells were frequently found on association with the metastatic tumors. From the results, it was concluded that malignant mixed tumor of the mammary glands metastasized to the infralunbar lymph node, abdominal cavity, lung and brain. In addition, the observation in this study supported two theories at the same time that the bone in malignant mixed tumor arises by endochondral ossification of the cartilage formed by the myoepithelial cells and arises by intramembranous ossification of stromal connective tissue or transformed myeopithelial cells. Solid carcinoma of the nasal epielia and granulosa cell tumor were also diagnosed in a mass of the nasal cavity and of the ovaries respectively.