Localized platelet aggregates have been observed on bileaflet mechanical heart valves (MHV) explanted from sheep. Such aggregates may be relevant to clinical thrombosis and thromboembolism, and may be a result of intravalvular flow phenomena. In this study, flow within the lumen of a fully transparent model bileaflet MHV, in the ‘mitral’ position of a pulsatile flow loop, was characterized using phase-locked particle image velocimetry (PIV). During each phase of the simulated cardiac cycle, velocities and vorticities within a central lumenal plane were analyzed. Observed flow phenomena included regurgitant jets, high-velocity gradients and vortices near model MHV surfaces corresponding to sheep explant MHV surfaces showing platelet aggregation. The shear stresses (for jets, velocity gradients and vortices) or recirculations resulting from these flow phenomena, observed in vitro, may promote platelet aggregation in vivo. The results demonstrate the advantages of using PIV with a fully transparent model to characterize intravalvular MHV flow.