Tracings of the respiratory pattern were obtained during three levels of activity at ground level, exercising at 20,000 and 35,000 feet and while shivering in the cold. From these tracings the instantaneous rate of flow of the air into and out of the lungs was calculated. The per-minute rate of ventilation and the maximum instantaneous rate of inspiration and expiration do not appear to be significantly altered in mild degree of work at these altitudes or when shivering;Using a low pressure chamber open parachute descent from 30,000 feet has been simulated. The degree of anoxia reached, which is considerable, increases the ventilation rate about two and one-half times. This stimulation is opposed by alkaline condition brought on by a reduced carbon dioxide tension in the blood;Hyperventilation while resting begins at approximately 15,000 feet breathing air, and about 38,000 feet breathing 100 percent oxygen. At about 37,000 feet the arterial oxygen saturation of the blood begins to fall below normal in spite of the inhalation of 100 percent oxygen. At 44,800 feet the partial pressure of oxygen in the alveolar air is at a critically low level;Although a virtual equilibrium exists between the barometric pressure and the sum of the partial pressures of oxygen, carbon dioxide and water vapor at extremely high altitudes while breathing 100 percent oxygen if at rest, it is concluded from work tests at 38,000 and 40,000 feet that this equilibrium breaks down in exercise. The transfer of oxygen from the lungs to the blood appears to depend on more than simple diffusion;It is concluded that the adequacy with which the anoxia is capable of stimulating respiration determines in a large measure a man's "ceiling". Anoxia in most cases is capable of maintaining adequate respiration at least to an altitude of 44,800 feet.