A series of in-vitro experiments conducted in our laboratory has shown that the echographic technique can be extended to quantify diaphragm motion in the total artificial heart (TAH). The data thus obtained have been utilized to compute a pump-characteristic-coefficient which incorporates the scale factor and surface area of the TAH pump diaphragm. This coefficient was found to be constant for all pumps of the same design. The product of the height of excursion of the diaphragm, as obtained by echocardiography, and the characteristic coefficient give the stroke volume of the pump and thus the cardiac output. With the availability of implantable echotransducers, it is possible to monitor the diaphragm motion. This technique has value in the non-invasive determination of stroke volume of total artificial hearts of the diaphragm type.