The CO2 elimination of humans, either under basal or non-basal conditions, is significantly decreased at a simulated altitude of 30,000 feet as compared with the sea level values. Two methods for measuring the CO2 elimination were used. One method involved collection of the expired gases in Douglas bags, the analyses being made at sea level with the Haldane gas analyzer. The 2d method employed a thermal .conductivity analyzer for CO2 in O2; the analyses and vol. measurements were made directly at altitude. The avg. decrease of CO2 elimination of non-basal subjects at altitude as measured by these methods was 8.6%. The avg. decrease for subjects under basal conditions was 6.4%.