The experiment investigated the effect of Selank, Pro-Gly-Pro and Pro-Gly-Pro-Leu on the intensity of redox reactions in immunocompetent organs (thymus and spleen) on the model of "social" stress. The intensity of redox processes was assessed by determining the intensity of lipid peroxidation (LPO) in immunocompetent organs (thymus and spleen) and catalase activity. "Social" stress, formed in the experiment, is accompanied by an increase in the peroxidation processes in immunocompetent organs, which contributes to the development of stress-induced functional disorders of the immune system. Under the influence of "social" stress, the activity of the investigated antioxidant enzyme in the thymus and spleen increased in comparison with the corresponding indicators in intact rats. Against the background of Selank, Pro-Gly-Pro and Pro-Gly-Pro-Leu administration under "social" stress, its pronounced corrective effect on lipid peroxidation rates is observed, as evidenced by a decrease in spleen and thymus tissue homogenates of male rats in the baseline level of TBA-reactive products, speed of spontaneous and ascorbate-dependent lipid peroxidation, and catalase activity.