The in vitro velocity and turbulent shear stress fields created by three new mechanical valve designs (size 27 mm) were studied in the aortic position under pulsatile flow conditions. The following valves were studied: Björk-Shiley Monostrut tilting disc, Omni-Carbon tilting disc, and Duromedics bileaflet. All three valve designs created low pressure gradients with effective orifice areas in the range of 3.10 to 3.90 cm2. Both tilting disc designs created major and minor orifice jets, which were asymmetric in size. The peak velocities of the major and minor orifice jets were, however, of the same magnitude (200 cm/sec). The Omni-Carbon valve created a more even flow distribution through the minor orifice compared with the Björk-Shiley design. Regions of stagnation/flow separation were observed immediately adjacent (ie, distal) to the minor orifice strut and the pivot guards of the Björk-Shiley and Omni-Carbon valve designs, respectively. The Duromedics valve created relatively centralized flow. However, a major portion of the flow occurred through the two lateral orifices. Regions of flow separation/stagnation were observed adjacent to the valve sewing ring in the area of the valve pivot (hinge) mechanism. All three valve designs did create elevated turbulent shear stresses, with peak values in the range of 1000 to 2000 dynes/cm2 and mean values in the range of 100 to 1000 dynes/cm2. Such elevated shear stresses could cause sublethal and/or lethal damage to cellular blood elements. In an overall analysis, these new-generation low-profile mechanical valves are hemodynamically comparable to the Medtronic Hall and St. Jude Medical mechanical valves and are superior to the older-generation mechanical valves. However, it is unlikely that these valve designs will eliminate the problems of thrombosis, thromboembolic complications, and hemolysis.