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Basal metabolic rate, BMR, was measured at sea level and at altitude in 5 non-athletic males, two non-athletic females and 6 champion runners. Related measurements included pulmonary minute volume, heart rate, respiratory frequency, respiratory quotient and blood pressure. There was a wide variation in individual responses. Basal metabolic rate increased in all: the range was from a few percent to as much as 50%. The two females spent 25 days at 3,800 m followed by 19 days at 3,090 m; in the latter period, BMR of one of them approached the control level. There were individual patterns of response in BMR, pulmonary minute volume and heart rate but no pat­tern could be related to sex, age or physical activity. Pulmonary minute volume was elevated throughout the altitude stay, generally involving increases both in tidal volume and respiratory frequency. The breathing pattern was much more variable at altitude than at sea level indicated by a greater stan­dard deviation of respiratory frequency. Our findings, viewed in the light of other studies, indicate that large increase in BMR and smaller changes inheart rate and blood pressure are transitory. Eventually BMR returns to oralmost to sea level values, heart rate and blood pressure can adjust to sea level values even at 5,300 m and only pulmonary ventilation remains high.