Argininosuccinate (ASA) is excreted in large amounts in the urine of patients with argininosuccinate lyase (EC 188.8.131.52) deficiency (ASLD). Previous workers have reported the presence of anhydrides of ASA in the urine of patients with this disorder. We have used 1H-NMR spectroscopy to investigate the presence of these compounds in untreated urine. Our results indicate that the anhydrides were absent from urine and are artifacts produced by the conditions prevailing in the methods of analysis previously used to investigate the urine from patients with ASLD. We have also attempted to reproduce the conditions that might promote the transition to the anhydrides in the urine from a patient with ASLD and in solutions of argininosuccinate. Resolution of ASA and the two anhydrides is possible and a spectrum of the urine from a patient with ASLD shows that the anhydrides are present at below 1% of the total ASA under normal physiological conditions.