To investigate whether or not amino acids affect the urinary excretion of purine bases and oxypurinol, a 12% amino acid solution was infused to 6 subjects who took allopurinol (300 mg) 6 h before the study. Amino acid infusion increased the urinary excretion and the fractional clearance of uric acid and oxypurinol and decreased the plasma concentration of oxypurinol. However, it affected neither the urinary excretion, the fractional clearance, the plasma concentration of oxypurines nor the plasma concentration of uric acid. These results indicate that amino acids affect the renal transport pathways of oxypurinol and uric acid but not those of oxypurines. In addition, it was suggested that the amino acid-induced increase in the urinary excretion of oxypurinol may be considered when allopurinol is administered to hyperuricemic patients with hypoproteinemia who have taken amino acids either orally or intravenously.