Abstract Self-contained artificial lung prototypes were implanted in 8 dogs and a calf for periods up to eight days. Ventilation and perfusion of the artificial organ are provided by pressures available in the pleural cavity and the pulmonary artery through the use of a silicone rubber ventilating envelope and an efficient, low-pressure manifold system. The three units with the largest membrane area (0.208 to 0.832 sq. m. had flow rates of 65 to 240 ml. per minute and oxygen transfer rates of 1.4 to 13.0 cc. per minute with room air ventilation. Design characteristics and the problems with implantation and long-term function are discussed.