If carbon dioxide is removed by an extracorporeal membrane lung ventilated with room air, the natural lung can be used for oxygen transport alone; we have demonstrated this in lambs by maintaining lungs "inflated" with 100 percent oxygen at constant pressure and removing all carbon dioxide through the membrane lung. This process is a variant of "apneic oxygenation" without its disadvantages, because the arterial pH, PCO2, and PO2 all remain normal. No nitrogen washout is needed. These studies were carried out in five lambs anesthetized and paralyzed for 24 hours. For carbon dioxide removal, blood from the subclavin artery was pumped through an extracorporeal membrane lung and was returned into the external jugular vein. For oxygen delivery, the lungs were inflated through a tracheostomy tube with 100 percent oxygen to a pressure of 5 cm. H2O. There was no significant change in arterial blood PO2 after perfusion had begun or at the end of the perfusion 24 hours later. The arterial PCO2 remained steady, and there was no change in acid-base balance. The functional residual capacity (FRC) and static lung compliance remained unchanged. The total dead space was 10 to 15 ml. All animals recovered and survived in good health. At equilibrium, alveolar nitrogen partial pressure was always equal to the partial pressure of nitrogen in the ventilating gas of the membrane lung and was the sole determining factor in controlling alveolar oxygen concentration. Direct measurement of pulmonary gas showed alveolar gas at the level of the carina.