The relationship between running intensity and the accumulation of hypoxanthine and uric acid in plasma was studied in four well-trained runners. The runners each performed several 800-m runs at different velocities, each run being performed on separate days. Venous blood samples were collected before and at regular intervals after the runs. The concentration of hypoxanthine and uric acid was determined with high performance liquid chromatography (HPLC) in plasma extracts. A marked increase in the plasma concentration of both hypoxanthine and uric acid occurred simultaneously at intensities corresponding to 110, 108, 115 and 107% of VO2max for subject a, b, c and d, respectively. The sudden sharp increase in plasma concentration of hypoxanthine and uric acid may indicate that at a certain level of running intensity ATP catabolism exceeds the rate of ATP regeneration from the normal metabolic pathways.