The effects produced by breathing high concentrations of oxygen for prolonged periods of time have been studied intensively by many investigators in a variety of animals including frogs, turtles, pigeons, mice, rats, guinea pigs, cats, dogs and monkeys.1 Practically all of these investigators have reported the occurrence of irritation, congestion and edema of the lungs and even death following long exposures. Because of the recent introduction and widespread use of apparatus designed to deliver 100 per cent oxygen it is highly important that similar well controlled experiments be done on man to determine the limits for safe oxygen usage in therapy and in aviation. Satisfactory experiments of this type have not yet been performed. Behnke2 reported that healthy men between the ages of 20 and 40 are unable to breathe 99 per cent oxygen for periods in excess of seven hours because of nausea, dyspnea and substernal soreness; complete