The aim of this study was to evaluate the plasma concentration of insulin like growth factor-2 (IGF-2) and its expression in tumor tissue of dogs with malignant mammary tumors. Fifteen dogs were used in the study. Eight dogs with mammary tumors constituted group 1, and 7 healthy dogs constituted group 2. Diestrus phase in dogs was determined with vaginal cytological examination. Metastasis was evaluated by ultrasonographic examination of abdomen and thorax radiography. Blood samples were collected from the dogs, plasma was separated and IGF-2 levels were measured with ELISA method. Tumors in group 1 were classified according to Clinical Grading System defined by World Health Organization and were removed by using complete unilateral mastectomy technique. The histological malignancy grading of tumors was made according tubule formation, nuclear pleomorphism, and number of mitotic cells by using Notthingham method. In histopathological examination, 4 carcinosarcoma, 1 intraductal papillary carcinoma, 1 carcinoma, 1 complex type carcinoma, and 1 mix type carcinoma were detected. In immunohistochemical investigation, IGF-2 expression was detected in mammary acinar epithelial cells, myoepithelial cells, and some vascular smooth muscle cells but not in ductal epithelial cells in tumor tissue. Compared to healthy dogs, the increase in plasma IGF-2 concentration in dogs with malignant mammary tumors was statistically significant (P<0.05).