Fourteen 1.4 m 2 hollow fiber and 23 1.5 m 2 spiral coil membrane oxygenators that were designed for total cardiopulmonary bypass in infants were evaluated. Operating procedures and toxicity were studied during total cardiopulmonary bypass and one hour of circulatory arrest at 15° C (rectal) in four monkeys. Oxygen and carbon dioxide transfer, water loss, production of emboli, transmission of pulse waves, and pressure-flow relationships were studied at three different blood flow rates, at hematocrits of 25% and 40%, and at 25° C and 37° C during partial venoarterial bypass in 33 dogs. Linear regression equations were calculated for oxygen and carbon dioxide transfer and for pressure-flow relationships at the different flows, temperatures, and hematocrits. Rated flow (Rf), defined as the blood flow rate at which inlet O 2 saturation of 65% increases to an outlet O 2 saturation of 95%, was calculated for each oxygenator at hematocrit 40 (Rf 40) or hematocrit 25 (Rf 25). The hollow fiber oxygenator had a priming volume of 140 ml, an Rf 40 of 800 ml/min, and an Rf 25 of 1200 ml/min. The spiral coil oxygenator had a priming volume of 208 ml, an Rf 40 of 1400 ml/min, and an Rf 25 of 1800 ml/min. At Rf 40 and inlet pCO 2 50 torr, the hollow fiber oxygenator transferred 8.9 ml CO 2 /100 ml and the spiral coil 5.7 ml CO 2 /100 ml. The performance of the two oxygenators differed, but both functioned very satisfactorily.