Peyer plaques of rats were studied by morphological methods in order to compare the structure of these organs with the present theories on their function. It was established that near to the follicules of plaques folds perpendicular to the intestinal passage are formed, which are able to direct a part of the bowel-contents into the direction of the follicules. Between the surface of the follicules and large intestinal villi recesses are formed, in which stagnation of the intestinal content takes place. As a result of this antigens and mitogens containing in the intestinal lumen can contact lymphoid cells of through the special epithelial cells the Peyer-plaques. The latter were recently studied by polarized light too, since earlier studies of the authors have shown that birefringence of these cells depends on their functional state. Authors disclosed that birefringence of the cells of the Peyer-plaques is significantly stronger, than that of the cells of spleen, lymph nodes, and thymus. Cells with increased birefringence are situated on the epithelial side of the capsule of follicules. Reported studies evidence, that Peyer-plaques are structures specialized for incorporation of antigens. Lymphoid cells of Peyer-plaques are activated by effects coming from the intestinal lumen.