Spontaneous canine mammary tumors were studied in bitches of diffe-rent breeds at an age ranging from 6-10 years. Twenty-two bitches were positive to mammary tumors and eight were negative. The mammary tumors were classified into three groups. The first group contained fibrosarcoma, which was detected in 9 out of 22 bitches. In this tumor, the mitotic activity was not obvious but the malignancy of the tumor was based on high cellularity, polymorphism and hyperchromatic nuclei. Some areas of the tumor showed metaplastic changes in which the fibrocytes showed the feature of chondrocytes. The second group of mammary gland tumors was classified as cystic adenocarcinoma, which was detected in 6 out of 22 bitches. In this tumor, there were dilated cysts, its wall was lined by cuboidal or columnar epithelial cells and the cysts were empty or filled with protein rich fluid or necrotic debris. In addition, in some areas of the tumors, the lactating acini may show degenerative and hyperplastic changes. The third group was classified as adenofibrosarcoma and was detected in 7 out of 22 bitches. The tumors showed proliferation of neoplastic cells of the acini and ducts and formed cystic and papillary structures, which were separated by highly proliferated fibrous strands.