l-Carnitine when injected in mice 30 min before an LD₁₀₀ of ammonium acetate (12 mmol/kg body weight, intraperitoneal) reduced mortality (100% survival with 16 mmoll-carnitine/kg) and prevented the appearance of symptoms of ammonia toxicity. Brain ammonia decreased in the animals givenl-carnitine. Ammonia decreased the levels of glutamate in brain; they were partially restored byl-carnitine, which also reduced the increase in brain glutamine in animals given only ammonia. The redox state of the brain was altered following ammonia intoxication. The ratio of lactate to pyruvate in the cytosol increased while that of glutamate to α-ketoglutarate in the mitochondria decreased. These ratios were partially restored byl-carnitine. The implications of these findings are discussed relative to the mechanism of ammonia toxicity.