The structure of a spleen of 110 white not purebred rats - males (10 intact, 50 control, 50 experimental) at chronic intoxication by sodium tetraborate and the after intoxication period - the 7, 14, 30 and 60 days were studied. Chronic impact of sodium tetraborate on an organism resulted into increase in the relative weight of a spleen in comparison with body weight that led to increase in weight coefficient of a spleen. Increase in the relative area of all functional zones (periarterial zone, the germinal center, mantle and marginal zones) of lymphoid slug of a white pulp is established. Decrease of an index a red/white pulp, as a result of increase of the area of a white pulp in response to chronic intoxication of an organism is revealed. During the after intoxication period as a result of decrease in immune tension there occured decrease of body weight and decrease in the area of a white pulp. During this period there was a reliable decrease of the area of lymphoid slug and decrease in the area of its functional zones (a periarterial zone, the center of manifolding, mantle and marginal zones).