In the present study, we estimated the levels of various forms of glutathione and the state of the glutathione antioxidant system in the subcellular fractions of the rat brain and in the liver during early postnatal ontogeny. Several groups of animals were studied, including 10-, 20-, and 30-day-old male Wistar rats, which allowed us to study different periods of brain maturation. It was shown that during the postnatal development of the rat brain from day 10 to day 30 the contents of both reduced and oxidized forms of glutathione decreased. In early ontogeny, when the activity of most of antioxidant enzymes is low, reduced glutathione may perform an antioxidant function. On the other hand, despite the decrease in the absolute value, the portion of GSH in the total glutathione pool increased and modified the redox state of the cells toward a more reduced condition. The decrease in the GSH level may be related to lower activity of glutathione reductase, which reduces oxidized glutathione, or to increased activity of the antioxidant enzymes, such as glutathione peroxidase and glutathione-S-transferase, which use GSH as a cofactor in their reactions.