Effect of mild intensity exercise on the serum levels of hypoxanthine was studied. Eighteen healthy subjects performed 2 to 4 bouts of 5 minutes walk load at different intensities. At the beginning, thirteen of them walked at intensity more than 80% of the maximum. The serum levels of hypoxanthine increased to the levels of more than 6 times of resting values showing a peak at 10 to 20 minutes after the completion of the walk load. In 62 bouts of the walk load by 18 subjects, statistically significant relationship was demonstrated between intensity of the walk load and increase in serum concentration of hypoxanthine at 10 minutes after the completion of the walk load with correlation coefficient of 0.556. The serum hypoxanthine levels were significantly increased by the walk load even at mild intensity between 41 and 60%. Increment in the serum hypoxanthine concentration also showed positive and statistically significant correlation with physiological cost index. These results suggest that the serum levels of hypoxanthine increase following mild as well as moderate to submaximal intensity of exercise, and its increment may be used as an indicator of energy balance in the muscle during exercise at mild to high intensity.