We studied three groups of animals: 10-, 20-, and 90-day-old or adult male rats. This age group corresponded to different periods of brain maturation, i.e., periods of cell growth, intense myelination, and complete maturation of the brain. We found that the formation of products of free radical lipid oxidation decreased during ontogeny, whereas the activity of the antioxidant system increased during maturation. The activity of cytoplasmic superoxide dismutase was lower in the brain of developing animals as compared to the adults. The activity of glutathione peroxidase in the cytoplasm also increased during maturation; however, some decrease in this activity was revealed in the 20-day-old rats. Glutathione peroxidase uses reduced glutathione as a source of H⁺. The content of reduced glutathione increased by 20% during the period studied. The activities of glutathione reductase in the cytoplasmic fractions were significantly higher in comparison with the activities in the mitochondrial fractions in all age groups investigated. The activity of the cytoplasmic enzyme reached its maximum on the 20th day, whereas mitochondrial glutathione reductase decreased its activity at this time point. During maturation, the total contents of nonenzymatic lipid-, and water-soluble antioxidants changed differently. Thus, a decrease in the activities of glutathione-dependent enzymes on the 20th day was accompanied by an increase in the total contents of lipid-soluble antioxidants. The maximal levels of water-soluble antioxidants were revealed on the 10th day of postnatal life.