It is shown that the subcutaneous injection to the rats in the age from 2 to 24 months during 5 consecutive days every month with 10 microg/100 g body weight of delta-sleep inducing peptide (DSIP) suppresses lipid peroxidation preventing the increasing of malonic dialdehyde level in rats tissues and plasma, possesses a powerful antioxidant effect, which is realized by means of the activation of different endogenous antioxidant defense system of cell and extracellular fluid, including high- and low-molecular regulators of free radical processes. DSIP exerts stimulating influence upon the superoxid-dismutese, catalase, ceruloplasmin activities as well as the level of nonenzymatic antioxidants--urea and uric acids, because during organism aging the antioxidant defense systems are being suppressed. DSIP increases the volume of tissues and blood endogenous antioxidant defense system mainly by means of enzymatic antioxidant system, especially during later ontogenesis.