Abstract: The present study was carried out with the aim of determining whether ultrasound could be a reliable diagnostic tool in the evaluation of canine mammary gland tumors as well as to detect regional lymph node metastases. A total of 28 adult bitches of different breeds with clinical evidence of mammary gland tumors were used in this study. The mammary glands and the regional lymph nodes were gently palpated prior to scanning. Ultrasonographic imaging was performed using a B-mode gray scale ultrasound machine (TOSHIBA Capasee) equipped with a 7.5 MHz curvilinear array transducer. The ultrasonographic appearance of mammary gland tumour tissue was variable depending on its size and its severity. Small areas of mammary tumour tissue appeared as ill-defined hypoechoic structures with acoustic enhancement artefact, and the larger areas appeared hyperechoic, sometimes with the presence of cysts. The spread of tumour tissue among the mammary glands was successfully detected by ultrasound, and ultrasound did not miss any tumour lesion of the mammary gland. The involvement of the regional lymph nodes (axillary and superficial inguinal) was also successfully detected by ultrasound. However, ultrasound could not truly determine whether the enlarged lymph nodes were due to metastatic spread of tumour tissue or due to inflammatory process. Ultrasonography would appear to offer potential to become a routine procedure in mammary tumour tissue detection in small animals.