EARLY attention to physiologic disturbances on ascent to high altitudes was directed to the effects of decreased partial pressure of atmospheric oxygen. The elimination of this factor with the use of modern oxygen equipment has disclosed new aspects of the situation no less serious in the development of full utilization and effectiveness of present and future aircraft in war and commerce. 1 When men are exposed to low barometric pressures equivalent to altitudes greater than 25,000 feet (7,500 meters), symptoms of decompression sickness frequently appear. These symptoms become commoner and severer with increases in altitude and physical activity 2 and are not due to inadequate oxygen content of the inspired air. 3 The commonest symptoms are joint pains (aviator's "bends"), thoracic symptoms ("chokes"), headache, abdominal gas pain, hyperventilation, muscular pain, dizziness and syncope; neurocirculatory postflight reactions also occur. 4 Any of these symptoms may prove disabling. In addition, defects in