With the use of the pulse duplicator built in our laboratory, the hydrodynamic characteristics of three sizes of the four commercially available mitral bioprostheses, Hancock, Carpentier-Edwards, Angell- Shiley, and Ionescu-Shiley, were studied and compared. A wide range of performance was found: for example, during pulsatile testing, at peak flow of 15 I/min (corresponding to a normal resting cardiac output) transvalvular gradients varied from as high as 20 mm Hg (Angell-Shiley) to 5 mm Hg (Ionescu-Shiley) in the 25 mm mounting diameter size. Effective orifice areas (EOA) are significantly different in valves of the same mounting size, e.g., at peak flows of 20 1/min, the Ionescu-Shiley 25 provides an EOA of 1.7 cm2 while the Angell- Shiley provides only 1.17 cm2. The EOAs of all bioprostheses have been found to increase with increasing flow (e.g., from 10-30 1/min peak flow, the Hancock 25 changed from 1.25 cm2 to 1.50 cm2). The Gorlin formula, as constituted for calculating the area of stenotic mitral valves, is inappropriate for prosthetic valves. But the discharge coefficients of the bioprostheses have been found to be around 1 when the planimetered area of the open valve orifice is determined at a given flow. By using this discharge coefficient, the Gorlin formula will give an excellent estimate of the true orifice area of mitral bioprostheses.