Designing an effective intravenous membrane oxygenator requires selecting hollow fiber membranes (HFMs) which present minimal resistance to gas exchange over extended periods of time. To evaluate HFMs, we developed a simple apparatus and methodology for measuring HFM permeability in a gas-liquid environment which has the capability of studying a variety of fiber types in any liquid of interest, such as blood. Using this system, we measured the O-2 and CO2 exchange permeabilities of Mitsubishi MHF 200L composite HFMs and KPF 280E microporous HFMs in water at 37 degrees C. The membrane permeability measured for the MHF 200L composite fiber was 7.9 X 10(-6) ml/s/cm(2)/cmHg for O-2 and 8.4 X 10(-5) ml/s/cm(2)/cmHg for CO2, and for the KPF 280E microporous fiber, 1.4 X 10(-5) ml/s/cm(2)/cmHg for O-2 and 3.2 X 10(-4) ml/s/cm(2)/cmHg for CO2. The permeabilities of the microporous HFMs were over two orders of magnitude less than what would be measured in a gas-gas system due to liquid infiltration of the pores, emphasizing the importance of measuring permeability in a gas-liquid system for relevant applications such as intravenous oxygenation. Furthermore, both O-2 and CO2 permeabilities of the microporous fiber were consistent with a liquid infiltration depth of only 1%. The O-2 permeability of the MHF fiber was found to be less than the overall exchange permeability ultimately required of our intravenous oxygenation device (K approximate to 1 X 10(-5) ml(STP)/s/cm(2)/cmHg). Consequently, the MHF 200L composite fiber appears unsuitable for intravenous oxygenation devices such as ours.