The ability to administer practically 100 per cent oxygen in the inspired air, economically, efficiently and comfortably, has opened a new field for oxygen therapy. Oxygen in lower concentrations is equally efficiently administered by an apparatus that recently has been described.1There is a place in therapy both for low and for high concentrations of oxygen. By administration or inhalation of oxygen in any certain percentage is meant the average amount of oxygen in the total inspired air. The amount of oxygen, after correcting for water vapor in the alveolar air, is between 5 and 6 per cent lower than whatever may be the percentage of oxygen in the inspired air. RATIONALE OF AND INDICATIONS FOR ADMINISTRATION OF ONE HUNDRED PER CENT OXYGEN The underlying physiologic principles and factors on which inhalation of 100 per cent oxygen is based, and the indications for its use, can be presented most