Subcutaneous injections of exogenous delta-sleep inducing peptide (DSIP) to rats aged 2- 24 months (100 μg/kg body weight, 5-day series of injections) have been shown to efficiently prevent oxidative damage to lipids and malonic dialdehyde accumulation in the tissues and blood plasma of rats of different ages. DSIP has a strong antioxidant effect mediated by the activation of various endogenous mechanisms of antioxidant protection both in cells and in the intercellular liquid containing high molecular weight and low molecular weight regulators of free-radical processes. DSIP has a stimulatory effect on the activity of superoxide dismutase, catalase, and ceruloplasmin, and affects the concentration of nonenzymatic antioxidants- urea and ureic acid; the physiological aging of the organism is accompanied by suppression of antioxidant protection mechanisms. DSIP increases the capacity of endogenous antioxidant protection systems in the tissues and blood, mostly by stimulating the components of the enzymatic antioxidant system, especially in late ontogenesis. © Pleiades Publishing, Ltd., 2011.