tension was related to the fetus's relative tolerance for hypoxia and the estimated and known levels of partial pressure of oxygen in the peripheral arterial blood of the fetus prior to delivery. The subsequent use of continuous positive airway pressure, positive end-expiratory pressure, and continuous negative pressure on the chest wall served to increase lung volume at end expiration, reduced the lung's tendency to collapse when the surface active material was deficient, and improved the efficiency of utilization of the supplemental oxygen concentrations. As a result of these efforts, there appears to have been a decrease in the duration of exposure of infants with severe hyaline membrane disease to 80 to 100% oxygen?