Mammary gland tumors in dogs rank second among all newly diagnosed tumors in this species. Research goalwas to study clinical/morphological characteristics of breast cancer in dogs with the use of ultrasound investigation. 34 dogs of different breeds and age groups with spontaneously emerging neoplasms of mammary glands were chosen for the study. To classify malignant tumors, the histological classification was applied. Complete examination of breast tumors was carried out with the help of clinical observation, ultrasound, radiologic, hematological and ultrasonographic methods applied. The largest percentage (23.5%) of animals with mammary gland neoplasms are mongrel female dogs. The breed predisposition was detected in dogs of the following breeds: Miniature Schnauzer - 11.9%, Standard Dachshund - 11.9%, American Bulldog, American Cocker Spaniel and Miniature Poodle - 8% each. Breast cancer was diagnosed in 29 animals (85.3%), 100% of those cases in the nodular form with microcalcifications in the tumor parenchyma. The horizontal orientation of growth is more often typical of breast cancer in the mammary gland pairs I and II at the initial stage of carcinogenesis, and is characterized by an inhomogeneous hypoechoic structure with sufficiently pronounced boundaries detected during the ultrasound investigation. The vertical orientation of growth is mainly typical of breast cancer in the mammary gland pairs III, IV and V from the very beginning of the disease. The ultrasonography usually shows: inhomogeneous hypoechoic structure containing inclusions in the form of cords or foci of a higher acoustic solidity; also clearly defined lateral acoustic shadows and dorsal echo enhancement; as well as unevenness and indistinctness of borders. The performed histological study within the current research revealed a multifocal, infiltrating epithelial neoplasm with distinct edges, surrounded by a large amount of fibrous tissue. The tumor consisted of a number of acini with a tendency of forming a network structure. The shape of cells was detected from round to irregular, the cytoplasm was found scarce, often with uneven borders, with the large central core, with chromophilic substance and 1-2 nucleoli. The cells were characterized by the pronounced anisocytosis and anisocariosis, and a great number of mitotic figures. Lymphatic invasion was observed. Ultrasound investigation of the mammary gland allows for performing percutaneous minimally invasive interventions and differential diagnostics of cystic and solid lesions of the mammary gland (96-100% accuracy), mastopathy and carcinoma conditions.