PREVIOUS experiments in this laboratory have demonstrated our ability to support dogs with extreme airway obstruction by means of an extracorporeal pump-oxygenator.1A prepulmonary bypass2was established in which venous blood was drained by gravity and then propelled through a membrane-oxygenator and returned to another systemic vein. Clinical application of this technique would be in infants with respiratory distress syndrome, most of whom are premature. The present experiments were designed to provide experience in dealing with subjects weighing about 2 kg (4.4 lb) and to determine whether complete airway occlusion for one hour could be tolerated by these small subjects while gas exchange was accomplished outside the body. The choice of a membraneoxygenator was necessitated by the fact that prolonged extracorporeal circulation would be required in the clinical application. Oxygenators with a blood gas interface are unsatisfactory3for prolonged perfusions probably due to protein denaturation at that