Although argininosuccinate is a product of the catalytic action of deficient argininosuccinate synthetase in citrullinemia, its concentration was found to be elevated in the urine of patients with type II citrullinemia. Urinary argininosuccinate was identified by two methods; its conversions to anhydride by boiling in an acidic solution and to arginine by the enzymatic action of argininosuccinate lyase. Oral administration of citrulline to patients with type II citrullinemia and control subjects increased urinary argininosuccinate levels. These phenomena are consistent with our previous findings on type II citrullinemia (Adv Exp Med Biol 1983;153:63-76,J Clin Biochem Nutr 1986;1:129-142), namely that renal argininosuccinate synthetase which plays a role in arginine synthesis is not deficient in patients with type II citrullinemia; and that serum arginine levels in patients with type II citrullinemia are rather higher than the controls, and increase after the oral administration of citrulline. The organ-specific deficiency of argininosuccinate synthetase in type II citrullinemia is further confirmed by this paper.