Flight students and Hospital Corpsmen were taken singly to simulated altitudes of 28,000 ft. to 38,000 ft. in a low pressure chamber and had their masks removed. While at altitude, the subject performed a simple task of card sorting and his reactions were noted and recorded. These activities were continued until unconsciousness was imminent at which time the mask was replaced and 100% O2 was administered. Blood O2 saturations were estimated throughout the procedure with a Millikan oximeter. Ambient O2 pressure was detd. with a continuous O2 analyzer. This procedure was repeated for 25 individuals at each of 4 altitudes: 28,000, 30,000, 35,000 and 38,000 ft. Times to imminent unconsciousness were 141 sec. at 28,000 ft., 98 sec. at 30,000 ft., 72 sec. at 35,000 ft. and 47 sec. at 38,000 ft. Avg. times of useful consciousness, as detd. by the appearance of first error in card sorting, were 100 sec. at 28,000 ft., 73 sec. at 30,000 ft., 46 sec. at 35,000 ft., and 35 sec. at 38,000 ft. The period of useful consciousness was found to be approx. 3/4 of the total time consciousness was retained. Blood O2 saturations averaged 64% of the appearance of first error, and 56% at imminent unconsciousness with very little variation between altitudes and the appearance of any particular anoxic symptom.