Injection of large amounts of ammonium salts leads to rapid death of animals. However, the molecular mechanisms involved in ammonia toxicity remain to be clarified. We have tested the effect of toxic dose of ammonium acetate on the activities of antioxidant enzymes in rat liver, brain, heart and erythrocytes. Acute ammonia intoxication resulted in rapid (within 11 min) decrease in superoxide dismutase, catalase and gluthatione peroxidase activities in liver and brain mitochondria and cytosol and in erythrocytes, but in increase of these enzyme activities in heart. Diminished activities of the antioxidant enzymes in liver, brain erythrocytes suggest that the systemic oxidative stress takes place, whereas their elevated activities in heart can be the adaptive reaction to oxidative stress in hyperammoniemia.