Localisation of the sentinel lymph node with methylene blue dye in bitches with mammary gland tumours Mammary gland tumours are the most common tumours that appear in bitches. The prevalence of the tumors varies from country to country because it depends on the percentage of castrated bitches. A number of factors contributing to the development of mammary gland tumours in bitches. The most important factor is hormonal stimulation by progesterone and estrogen. The protective effect is achieved by castration, but castration after the first heat does not last for life. In bitches with malignant neoplasia of the mammary gland, there is a change in lymphatic drainage due to the release of lymphangiogenic factors, and the lymph drains into the contralateral lymph nodes. Due to changes in lymphatic drainage, mastectomy is not recommended according to existing data on lymphatic drainage. Along with the tumor and the affected mammary complex, the associated lymph nodes are also removed. For the purpose of complete diagnosis and reduction of the complications of unnecessary lymphadenectomies, localization of the sentinel lymph node is being carried out by different methods. Sentinel is the first lymph node into which the tumor will drain. If the pathohistological examination shows that there are no metastases in the sentinel, there is a high probability that the tumor has not spread systemically. Methylene blue is an agent widely used for therapeutic and diagnostic purposes. It can be used to localize the sentinel lymph node because it is easily available, cheap and has no major side effects. However, the sentinel lymph node can be located deeper in the tissue or be smaller in size, which will make it difficult for the surgeon to localize it intraoperatively. In such cases, in human and veterinary medicine, better results have been achieved with a combination of methylene blue and radiocolloids.