To exercise at high altitude means working in an environment with reduced atmospheric pressure. The oxygen tension of the inspired air is therefore decreased, that is, there is atmospheric hypoxia. Exercise increases oxygen requirements which must now be met in the face of this decreased oxygen driving pressure. The initial handicap is less complete oxygenation of blood within the lung. In an effort to preserve oxygen delivery, a greater volume of blood is circulated, that is, cardiac output is increased. However, this pattern of compensation is only temporary. Within days, hemoconcentration increases the oxygen-carrying capacity of the blood, and as a consequence, less cardiac output is required to maintain oxygen delivery. In fact, cardiac output decreases to levels lower than existed prior to ascent. This reduction in cardiac output results primarily from a decrease in stroke volume due to less venous return secondary to the smaller blood volume produced by hemoconcentration. The hypoxia of high altitude produces sustained stimulation of the sympathetic nervous system. Initially, this increases heart rate, but, with time, the responsiveness of the heart decreases, so the initial tachycardia may not be sustained. Other consequences of sympathetic stimulation include an increase in resting metabolic rate, a shift away from glycogen toward free fatty acids as primary energy sources, and bone marrow stimulation to increase red cell production. The parasympathetic nervous system may also be stimulated at high altitude, which may explain the reduction in maximum heart rate. Upon arrival at high altitude, aerobic working capacity is reduced. Although this may or may not be attenuated following adaptation, endurance capacity does seem to improve. Several parallels therefore emerge between adaptation to the hypoxia of high altitude and adaptation to the struggle for oxygen created by exercise training at low altitude. Sympathetic stimulation is common to both forms of hypoxic stress, and similar responses, particularly metabolic, result. Not surprisingly, then, exercise training provides an advantage to adaptation to high altitude.