[en] Changes have been revealed in the function of cyclic GMP system of thymus and liver of irradiated (8 Gy) mice. In the thymus the cGMP level increased during the first 60 min following irradiation. In the liver the concentration of cGMP exhibited two peaks: 30 min and 24 hr after irradiation. The changes observed in the cGMP level are connected with the increased guanylate cyclase activity of thymocytes and liver of irradiated mice and, less likely, with changes in the activity of cGMP phosphodiesterase of these tissues