The Travenol TMO Total Bypass Membrane Oxygenator-Pediatric satisfactorily transfers oxygen and carbon dioxide under standardized conditions of operation during total high-flow bypass at normothermia or moderate hypothermia. This oxygenator has not been used at rates of flow less than 1 L/min or at temperatures below 23 degrees C. The current technique requires a venous and arterial pump with 5% recirculation to assure that pressure in the blood phase does not exceed the pressure above which blood may seep through the adhesive seals into the gas envelope. Conventional methods of perfusion were modified for use at profound hypothermia (18 degrees C) with low-flow bypass or circulatory arrest. Clinical experience with 22 children, preceded by six laboratory experiments, indicates that satisfactory control of arterial gases can be achieved and maintained.