The shift of prooxidant-antioxidant balance in side of prooxidants was revealed in rat liver mitochondria and in microsomes and in blood plasma in response to single irradiation (dose 8 Gy). The shift was more expressed in animals with nutrition unbalanced on animal proteins and antioxidant vitamins. In the main it was explained by the initially reduced activity of enzymatic antioxidant system and especially Se-dependent glutathione peroxidase activity. The apply of food addition from Aronia melanocarpa fruits had delayed lipid peroxidation activation in irradiated animals but practically had no effect on activity of enzymatic antioxidant system. The established essential decrease of Se-dependent glutathione peroxidase activity under unbalanced diet is considered the most crucial point in the maintenance of enzymatic antioxidant system reliability in irradiated animals.