Two different mechanical heart valves with annulus diameters 21-29 mm, (five Björk-Shiley monostrut tilting disc valves and five Duromedics bileaflet valves) have been tested in pulsatile flow in the mitral position of a mock circulation. Reflux, pressure, and orifice area have been measured while cardiac output was varied between 2 and 6 1 min-1. Insufficiency, mean orifice area, discharge coefficient, and performance and efficiency indices have been calculated. Mean values of insufficiency for the Björk-Shiley monostrut valves varied between 4.8 and 17.2% while the corresponding values for the Duromedics valves were in the range 6.1-17.3%. Mean values for orifice areas of the Björk-Shiley monostrut valves increased with the larger valve sizes from 101.1 to 210.2 mm2; for the Duromedics valves the area range was 134.5-262.9 mm2. Because of the larger orifice areas the values of discharge coefficient and performance index for the Duromedic valves were higher than those for the Björk-Shiley monostrut valves. As the insufficiency of the two mechanical valves was similar, and the orifice area of the bileaflet valves was greater than that of the tilting disc valves, Duromedics valves gave higher valves for the efficiency index, which varied between 0.31 and 0.39; for Björk-Shiley monostrut valves the index varied between 0.24 and 0.28 under the same test conditions. This hydrodynamic in vitro comparison of mechanical heart valves showed that the Duromedics bileaflet valves were superior to the Björk-Shiley tilting disc valves.