The research reported herein has been an attempt to produce a trileaflet aortic valve prosthesis using a synthetic cusp material with mechanical properties similar to the natural tissue. Natural leaflets exhibit high compliance and pronounced anisotropic behavior. Using the stress-strain behavior of natural tissue as a guide, a synthetic composite material was formed which resembles the native tissue in terms of anisotropy and compliance. The composite is made of a thin knitted dacron fabric coated with a layer of a recently developed, non-thrombogenic polyurethane. The dacron knit simulates the collagenous region of the native material, providing the distinct mechanical properties. The polyurethane provides a continuous, non-permeable, blood compatible coating. A significant portion of the research involved selecting methods to describe the geometry of the aortic valve and to translate the geometry into techniques for forming valves. All the valves were formed in the open position. The cusps are described in the open, excised configuration as semi-ellipses with a ratio of semi major to semi minor axis of 1.3. A series of all-polyurethane valves were formed intact in mounting tubes, via a solvent casting process, for use in ventricular assist devices where the performance criteria are less stringent. One of these valves includes physiologically shaped simulated sinuses. Preliminary mock loop testing of this valve indicates excellent performance. The intact polyurethane valves are attractive candidates for incorporation into the left ventricular bypass pumps under development at Rice and Baylor. Trileaflet valves were constructed from the anisotropic composite material. In a pulsatile mock circulatory loop, the composite material failed. Separation occurred between the dacron and polyurethane fabric on portions of at least one cusp, rendering each valve incompetent. At the present state of development, the composite material made of these particular components is unsuitable for a prosthesis.