The types of adaptational reactions, masses of the thymus, the spleen and the adrenal glands as well as the thymus histology and morphometry were studied in 79 healthy male rats which received one kind of medicines (sulfalen, pentoxyl, aspirin or ibuprophen), and in 20 animals as a control group. It was established that a quiet activation occured in 24% of animals, an increased activation – in 22%, an orientation one – in 20%, a defective adaptation – in 23%, stress – in 6%, and an overactivation – in 5% of rats. Changes in the immunocompetent organs, the adrenal glands and the peripheral blood were typical for all types of the nonspecific general adaptational reactions. The data obtained evidence that the adaptive reactions of both a quite and an increased activation should be considered as a contributory factor for body resistance as they are associated with mass enlargement, morphological signs of an activation of the organs of the immune system, and prevailing the mineralocorticoid function of the adrenal gland over the glucocorticoid one. Morphological changes in the thymus in stress reactions, in overtactivation and a defective adaptation are believed to be the background of many pathological processes.