The purpose of this study was to know the effect of short termed fasting on the usage of metabolic energy sources and the metabolic differences between non-athletic and athletic subjects. Subjects were divided into non-athletic and athletic group and exercise was loaded on both groups after feeding and fasting. Exercise was loaded by a treadmill running at the speed of 8 km/hour for 30 minutes in both groups. The experiment yielded following results. In the fed state, the level of plasma FFA increased markedly after 15 and 30 minutes of exercise compared with it's level of pre-exercise period in both groups. In the fated state, the level of plasma FFA in non-athletic group increased steadily according to the duration of exercise, while it's level in athletic group showed no changes. At pre-exercise period, the level of plasma FFA was higher in fasted state than fed state. Immediately before the exercise and 15 and 30 minutes after the exercise, blood for the determination of plasma free fatty acid (FFA), glucose, triglyceride (TG) and cholesterol was sampled from antecubital vein, and simultaneously heart rate was measured. In the fed state, the level of plasma glucose was increased mildly according to exercise, and in the fasted state it's level increased according to exercise in both groups also. In the fasted state, the level of plasma TG was lower than that in the fed state. The level of plasma TG and cholesterol in the fed state was no changed by the exercise from the pre-exercise period. The level of plasma cholesterol in athletic group had tendency to lower than that in non-athletic group. Heart rate increased markedly according to exercise in both groups, but the athletic group's increasing rate of heart rate was lower than the non-athletic group's heart rate increased according to exercise and athletic groups heart rate increased early period of exercise, but did not change during latest post-period of exercise.