A nonthrombogenic left-heart bypass system employing a pulsatile pump, porcine aortic valves, and polyurethan-polyvinyl-graphite-tetrahydrofuran-coated tubing to bypass the cross clamped thoracic aorta was evaluated in animal experiments. This system provided accurate control of proximal and distal aortic pressures and better tissue perfusion than nonpulsatile bypass techniques. Intraoperative and postoperative hemorrhage was markedly reduced because heparinization was not required. This nonthrombogenic pulsatile bypass would appear to be the method of choice to permit prolonged cross clamping of the thoracic aorta.