The content of homocarnosine, GAMA, histidine, glutaminic acid and activity of glutamate decarboxylase were studied in four regions of rats brain: cerebral hemispheres, midbrain, diencephalon and cerebellum, in norm and under hyperoxia. A considerable decrease in the content of homocarnosine, GAMA and histidine is observed in all the studied regions of the rat brain in the convulsion stage of oxygen poisoning. A decrease in the glutamate decarboxylase activity is the reason for a drop in the GAMA content. Homocarnosine in the brain is bound functionally with the GAMA level.