Abstract The oxygenating efficiency of the 1.0 m. 2 General Electric-Peirce and the 1.5 m. 2 Travenol membrane lungs was studied using a venoarterial bypass circuit in dogs. Oxygen transfer ( o 2) and Sa o 2 values were compared at consecutive increases in blood flow ( ). Each lung demonstrated, within limits, a direct relationship between and o 2. An inverse relationship between and Sa o 2 occurred at flows between 0.75 to 1.0 L./min./m. 2 and 0.835 to 1.0 L./min./m. 2 in the 1.0 m. 2 (General Electric-Peirce) and the 1.5 m. 2 (Travenol) lungs, respectively. These data reflect the inappropriateness of using o 2 in describing the performance of an oxygenating device. Rated flow, which is the maximum flow at which an oxygenating device will maintain an Sa o 2 of ≥ 95% when presented with blood having an Sv o 2 of 60 to 75% and a hematocrit of 40 to 42%, more completely reflects the capability of an oxygenating device.