A device for assistance of the left side of the heart in acute failure is placed in the paracardiac position. The blood is drained into the apparatus through the apex of the heart and is reinjected into the ascending aorta by an artificial pump. The flow of blood within the prosthesis and at the level of the inlet and outlet values is studied by a system of lighting in thin layers. Several types of original valve have been tested but the most satisfactory results were obtained with a classical bead valve.