Extreme altitude presents an enormous physiological challenge to the human body because of severe oxygen deprivation. The American Medical Research Expedition to Everest was specifically designed to study man under these conditions, and successfully obtained physiological data above 8000 meters, including a few measurements on the summit itself. The results show that man can tolerate the extreme hypoxia only by an enormous increase in ventilation, which results in an alveolar partial pressure of carbon dioxide of 7.5 torr on the summit and an arterial pH of over 7.7. Even so, the arterial partial pressure of oxygen is apparently less than 30 torr, and maximum oxygen uptake is about 1 liter per minute. Additional measurements of ventilation, blood physiology, and metabolic and psychometric changes clarified how man responds to this hostile environment.