The purines which were previously noted to increase in the urine during muscular exercise have been identified as hypoxanthine and xanthine. Quantitative studies on 11 human subjects showed that the magnitude of the increase in excretion of hypoxanthine varied greatly and reached up to 20-fold in some cases. The increase in the excretion of xanthine was less pronounced with the maximum about threefold. xanthine and hypoxanthine excretion Submitted on August 8, 1963