Maximal aerobic power is the highest peak oxygen uptake that an individual can obtain during dynamic exercise using large muscle groups during a few minutes performed under normal conditions at sea level. In most subjects maximal aerobic power is limited by the central circulation. It is obvious that stroke volume is one very important factor. Another important one is the oxygen content of the arterial blood. If the oxygen content of the arterial blood is increased the maximal aerobic power and physical performance is increased. If it is decreased--except when there is an acute volume expansion--or if the heart rate is depressed the circulation can not compensate for it and the maximal aerobic power is decreased. Certain mainly external factors like drugs or induced hypothermia influences on circulation, thus reducing maximal aerobic power and physical performance. Despite all these possible ways of influencing circulation during maximal exercise, it is interesting to note that repeated measurements of maximal aerobic power show a fairly low variation from day to day.