To identify whether muscular exercise increases the plasma concentration of uridine and of purine bases, the effect of rigorous muscular exercise was determined in five healthy men with a bicycle ergometer. Twenty-five-minute muscular exercise at 65% maximum O2 consumption increased the concentration of uridine, purine bases, and inorganicphosphate in plasma and of NH3 and lactic acid in blood. These results suggest that exercise-induced excessive adenosine triphosphate (ATP) consumption enhanced not only purine degradation but also pyrimidine degradation (uridine triphosphate [UTP]-->uridine diphosphate [UDP]-->uridine monophosphate [UMP]-->uridine) in exercising muscles.