Morphological transformations occurring in the thymus and lymph nodes are studied in Wistar rats after irradiation of the thymus projection area and transcutaneous irradiation of peripheral blood with a low-energy helium-neon laser during a 7-day period. Irradiation of the thymus projection area stimulates lymphopoiesis in T-dependent zones of lymph nodes, while irradiation of peripheral blood increases lymph flow through lymph node parenchyma and activates B-dependent zones.