The adjustments of the cardiovascular system to muscular exercise were studied in dogs running on a treadmill at different speeds and at the incline of +10%. The cardiac output, measured by the thermodilution method, increased with increasing O2 consumption, reaching 520 ml/kg min at an O2 consumption of 70 ml/kg min. At still higher metabolic levels the cardiac output remained constant and the O2 consumption increased by an increase of the arteriovenous O2 difference only, which was calculated to attain 17 vol % at the highest O2 consumption value reached in this study, 90 ml/kg min. The increase of the cardiac output was mainly due to increase of the heart rate, whereas the average maximum increment of the stroke volume was about 30% only. The mean arterial and the central venous blood pressures increased with exercise. The time course of the adjustment of the cardiac output was measured after the exercise of varied intensity had been abruptly begun or stopped. Both for start and recovery the “half reaction time” was about 20 sec; after 1 min no further measurable change of the cardiac output was detectable. cardiac output and O2 consumption at increasing metabolic levels; heart rate and stroke volume in running dogs recovery following exercise Submitted on July 15, 1963