In this study, we investigated the activities of several antioxidant enzymes during the postnatal development of the brain and liver of rats. Experiments were performed on male rats of different ages, viz., 5, 10, 20, 30, and 90 days, so that the different periods of the brain development could be investigated. The activity of the enzymes of the antioxidant system (superoxide dismutase, catalase, and glutathione peroxidase) in the immature brain was found to be lower than in the brains of adult animals. Cytoplasmic superoxide dismutase was an exception: its activity declined along with development in both brain and liver. High activity of antioxidant enzymes on day 5 of postnatal development in rats was found, which may be associated with the adaptation to the environment with increased oxygen content. Our results led to the conclusion that the formation of the antioxidant system in the postnatal development of the brain is accompanied by redistribution of the enzyme activity between subcellular fractions, as well as changes in the contributions of the main pathways of Н₂О₂ elimination: the activity of glutathione system enzymes increases and the catalase activity decreases.