1. The microporous membrane oxygenator has a maximum priming volume of 400 ml. and transfers up to 250 ml. of O2. 2. CO2 transfer is excessive when ventilating the oxygenator with 100% O2. CO2 should be added to the ventilating gas. 3. The oxygenator has performed satisfactorily in 90 cases of cardiac surgery with perfusion time up to nine hours. 4. Precautions are described to prevent possible accumulation of water vapor in the oxygenator.