Space physiologists have an interest in upper body exercise since in the weightless state astronauts do a substantial amount of work with their arms and hands. Upper body exercise elicits a peak oxygen uptake approximately 70% of that obtained during lower body exercise; in addition, upper body exercise requires a greater oxygen uptake at a given power output than lower body exercise. Therefore, when performing exercise at a given power output, both the absolute and relative (% of peak V̇O2) exercise intensity is greater during upper body exercise. Although cardiac output responses for a given oxygen uptake are similar, the heart rate, blood pressure and total peripheral resistance responses are greater, and the stroke volume responses are lower at a given oxygen uptake during upper than lower body exercise. Body temperature responses to both exercise types are similar, but the temperatures are achieved by different heat exchange mechanisms. During upper body exercise, there is a greater reliance on torso dry heat loss for temperature regulation. Exercise training programs can improve aerobic exercise capabilities for the upper body, but there are minimal cross-training benefits between the arms and legs. Space physiologists and engineers in the manned space program should consider the unique physiology associated with upper body exercise for: (a) assuring that astronauts are prepared to perform mission related tasks; (b) developing effective exercise countermeasures programs; and (c) engineering of adequate life support systems.