The effects of sodium citrate supplementation to improve aspartic acid supply in a 5-year-old girl with argininosuccinate lyase deficiency were studied over a period of 5 years under constant and total parenteral nutrition. Daily increasing doses of sodium citrate (0-8 mmol/kg) were continuously infused i.v.. Her standard therapy with arginine hydrochloride (4 mmol/kg/day) and sodium benzoate (200 mg/kg/day) was continued. Serum citrulline concentrations were reduced by citrate supplementation (161.8-41.7 mumol/l). Correspondingly serum concentrations of argininosuccinate and its anhydrides rose (270-458 mumol/l), positively correlated with the doses of sodium citrate. Renal elimination of argininosuccinate did not change measurably. There was no improvement in plasma ammonia concentration, which remained between 88 and 136 mumol/l. A long-standing hepatomegaly did not improve. The patient developed a significant alkalosis with blood pH-values increasing up to 7.55. The alkalosis lead to a reduction in renal ammonium elimination by at least 5.3 mmol/kg per day; the portion of ammonium-nitrogen decreased from 54.2% to finally 9% of total urinary nitrogen. Nevertheless total urinary nitrogen elimination, as measured by pyrochemiluminescence, improved by 24.5% during the 5 days and rose from 2065 mg nitrogen/day to 2570 mg/day. This improvement could not be explained by the metabolites determined and corresponded with an increase in chemically undefined nitrogen excretion, which rose from 0% to 65.9% of total urinary nitrogen. Further investigations are necessary to elucidate the nature of these unexplained nitrogen-containing compounds. CONCLUSION. Sodium citrate supplementation improved renal nitrogen elimination in a patient with argininosuccinate lyase deficiency in the observation period of 5 days by up to 24.5%.