Increasing partial pressure of oxygen by hyperbaric oxygenation improves the efficiency of membrane oxygenators sufficient to provide almost total respiratory support with membranes of practical sizes. The oxygenating capacities of the Teflon-Peirce, Landé-Edwards and G.E.-Peirce membrane oxygenators were tested using surface areas of 1 to 3 m2, flow rates of 500 to 2,000 ml/ min, and ambient pressures from 1 to 3 ATA (atmospheres absolute) in the hyperbaric chamber. For every 3 m2-membrane tested at 2 ATA or more the amount of oxygen in the blood leaving the oxygenator approached its maximum oxygencarrying capacity. The Teflon-Peirce membrane decreased in efficiency with increasing flow rates and is unsuitable for clinical use. The Landé-Edwards (3 m2) and the G.E.-Peirce (2 m2) at flow rates of 2,000 ml at 2 ATA were capable of transferring oxygen at the rates of 200 and 240 ml/min, respectively. It was not necessary to pressurize above 2 ATA to achieve efficient use of the membrane oxygenators tested.