Although Mechanical Heart Valves (MHVs) are unrivaled in terms of durability, their success is still limited due to thromboembolic, or clot inducing, complications. It is hypothesized that thromboembolic complications are initiated and maintained mostly by the non-physiologic flow patterns through the valve that induce platelet activation and aggregate them in the shed vortices generated in the valve's wake to produce small blood clots (thromboemboli). The popular bileaflet design has recently come under scrutiny for its relatively high rates of thromboembolic complications, leading to a renewed interest in the slightly older monoleaflet design. In this study, the leading St. Jude Medical bileaflet MHV was compared with a Medtronic Hall monoleaflet design. A non-invasive in vitro technique, Digital Particle Imaging Velocimetry (DPIV), was utilized to measure two dimensional displacement and velocity vector fields of fluid moving distal to a valve in a recirculation flow loop driven by peristaltic and pulsatile flow pumps. Computational Fluid Dynamics (CFD) was also employed in two dimensional geometries of the two valves to compare with the DPIV results. The results of the measured flow fields indicate high levels of shear stresses in the bileaflet valve design, generated by the jets during flow passage through the valve major and minor orifices, indicative of flow areas where platelet activation may occur. Distinct shed vortices were measured in the wake of the bileaflet valve, and a large recirculation zone characterized the flow field distal to the monoleaflet valve. Shear stress history of distinct particle trajectories were computed in both the CFD and DPIV, and are to be quantified in order to predict platelet activation levels within 'hot spot' regions in the valve flow fields. A novel approach was also applied for the DPIV measurements to construct a three dimensional presentation of the flow fields past the valves. Ultimately, results of this study may contribute to the continuous design modifications of the MHVs with the intent of minimizing post operation complications.