changes in immediate memory for objects, sorting efficiency, perceptual speed, efficiency of routine mental work, and concentration resulting from 1, 10, 13, 18 and 24 mon, of stay at an altitude of 4,000 m were determined in the case of 25 healthy male subjects. Sea-level performance after return from altitude was also recorded. Immediate memory remained unchanged at altitude. Concentration was the only function which, after recording a decline within the first month, continued to operate at the lowered level throughout the stay. Efficiency of remaining cognitive functions deteriorated during the first month of altitude exposure but recovered thereafter possibly due to acclimatization. Return sea-level scores were pretty close to original baseline levels.