To examine whether branched-chain amino acids affect the plasma concentration of uridine, we administered branched-chain amino acids (L-isoleucine, 2.85 g, L-leucine 5.71 g, and L-valine, 3.43 g) orally to 6 healthy subjects. Plasma uridine and glucose decreased by 44% and 12%, respectively, together with an increase in plasma isoleucine, leucine, and valine 90 minutes after administration. However, branched-chain amino acids did not affect the plasma concentration and urinary excretion of purine bases (hypoxanthine, xanthine, and uric acid) and uridine or the plasma concentration of insulin, glucagon, and cyclic adenosine monophosphate (cAMP). Since small amounts of regular insulin, which were found to decrease plasma glucose more than the amino acids, did not decrease the plasma concentration of uridine, these results suggest that plasma uridine was decreased by a direct effect of the branched-chain amino acids on the cellular uptake and/or release of uridine.