Aerobic power in prolonged exercise is related with the capability to continue exercise of an intensity corresponding to a fairly high percentage of V02 max besides the amount of V02 max itself. In the present study, the authors were led to the conclusion that the well-trained had excellent capability as regards physical resources and high adaptability to exercise and, accordingly, they showed more rapid acceleration of physiological functions than the untrained and could maintain high efficiency for oxygen supply through the whole course of exercise. Moreover, the increase in blood sugar content during exercise was marked in the trained, although exercise was considered to augment the uptake and consumption of sugar, but the increase in lactate content was comparatively small and that in serum FFA content was also rather low. Although the relative physiological work load seemed to be similar in both trained and untrained, the level of oxygen intake and the blood content during exercise were distinctly higher in the trained than in the untrained. These findings seemed to suggest the excellent aerobic power in the trained was helpful in augmentation of sugar oxidation.