An intravenous membrane oxygenator is being developed to supplement oxygen and carbon dioxide exchange in patients with temporary and potentially reversible lung failure in either a lung transplantation setting or in cases of acute respiratory distress from multiple causes. Our device incorporates a pulsatile balloon that is centrally located and around which are mounted microporous hollow fibers. Oxygen is vaccuumed through the fibers with resultant gas exchange. The rhythmic pulsations of the balloon enhance cross-flow and three-dimensional convective mixing at the blood-fiber interface and thus promote more efficient oxygen-carbon dioxide exchange. Seven intravenous membrane oxygenator prototypes have been designed and fabricated. Modifications in design have led to a progressive improvement in gas flux. Gas exchange performance measured in vitro and with both saline solution and fresh ox blood have shown gas exchange as high as 203 ml/min/m2 for oxygen and 182 ml/min/m2 for carbon dioxide. In vivo dog experiments with the device positioned in the inferior vena cava and right atrium have shown over a 50% increase in oxygen flux with balloon activation versus the static situation without changes in hemodynamics. The size of the prototype tested in animals can be scaled up fivefold for anticipated human trials. Our results indicate that our intravenous membrane oxygenator prototypes now under development may be an alternative to extracorporeal membrane oxygenation in the treatment of temporary respiratory failure.