OBJECTIVE the aim of our study was to identify of structural transformations in the thymus and lymph nodes at the early stages of the development of an experimental prostate cancer. METHODS Experiments were performed on 60 adult male CBA mice weighing 28-30 g. An experimental model of a prostate cancer was created by inoculating a diluted cell strain of Ehrlich Ascites Carcinoma cells into the prostatic parenchyma through midline laparotomy under ether anesthesia. The animals were divided into three groups of 20 each: 1) control (intact animals) 2) animals on the 5th day of experimental prostate tumor growth and 3) animals on the 18th day of experimental prostate tumor growth. The pathological examination of the prostate, thymus, and pelvic lymph nodes was performed. The selection of structural components and cellular elements in the thymus and lymph nodes was carried out according to the International histological nomenclature. RESULTS According to the results, there was an increase in the percentage of connective tissue elements and glandular tissue increased in the thymus and the number of immunoblasts increased on the 5th day of the experiment. However, disorganization of the thymus structure was detected on the 18th day as well as an increase in the cortical substance, the number of epithelial reticular cells and decrease in the number of immunoblasts. On the 5th day, paracortical hyperplasia, sinus histiocytosis and activation of the blast transformation were detected in lymphatic nodes. On the 18th day, metastases, follicular reaction and activation of the transport function were determined. CONCLUSIONS Our data suggest that the accidental thymus involution and impairment of transport and immune functions of pelvic lymph nodes in experimental prostate cancer. The severity of disturbances depends on the stage of prostate carcinogenesis.