The content of ammonia, glutamine, glutamic acid was measured in the brain, liver, heart, spleen, kidneys, skeletal muscles and blood rats exposed to a 4 atm oxygen atmosphere and during aftereffects. The hyperoxic atmosphere resulted in an increase of ammonia and glutamic acid and in a decrease of glutamine in the tissues. The return to the norm of the compounds occurred slowly and nonuniformly, lasting for 40 to 60 posthyperoxic days.