PURPOSE: to evaluate experimentally the features of structural and functional changes in lymphoid organs under the conditions of oral administration of high doses of sodium selenite and after its termination. MATERIAL AND METHODS. The study was conducted on 30 sexually mature male Wistar rats, weighing 200-220 g. The morphometric research of the thymus, group lymphoid nodules, mesenteric lymph nodes, spleen, and laboratory investigation of blood were performed. The animals were divided into 3 groups depending on the nature of the study. The first (control) group consisted of intact rats. Animals of groups 2 and 3 were orally administered sodium selenite, at a rate of 5 mg/kg of mass for 5 days. Morphofunctional studies of lymphoid organs in group 2 were performed on day 6, in group 3 – on day 14. RESULTS. The degree of severity and directional effect of structural and changes are determined by the functional specialization of the studied organs. CONCLUSION. Signs of endotoxicosis indicate evident structural and functional changes in the lymphoid organs after five times administration of sodium selenite. Canceling the administration helps to improve the organism's reserve capacity and adapt to altered metabolic processes.