Résumé : A newborn infant affected by argininosuccinic aciduria was treated with a hypoproteinaemic diet and survived up to the age of three months. Post mortem biochemical studies have confirmed the absence of argininosuccinase activity in kidney, liver and brain and a marked storage of argininosuccinic acid in the brain. The histological study of the central nervous system shows no Alzheimer type II cells; there is neither demyelination nor spongiosis. Such results are totally different from those recorded in untreated cases; they represent very likely the effects of the diet which has controlled the hyperammoniaemia and prevented a deficiency of arginine.