Objective: The optimal orientation of a bileaflet mechanical valve for tricuspid valve replacement (TVR) has not yet been determined. The aim of this study was to use fiberoptic cardioscopy to evaluate the effect of orientation of a mechanical valve implanted in the tricuspid position on bileaflet mechanical valve behavior. Methods: Twelve pigs (50–59kg) underwent TVR with a St. Jude Mechanical Heart Valve (25mm standard cuff model) after cardioplegic arrest. The mechanical valve was implanted horizontally in six pigs (Group H), and vertically in another six pigs (Group V). The heart was perfused with pellucid Krebs–Henseleit solution in situ and the mechanical valve behavior was observed with a fiberoptic endoscope during different heart rates (HRs) induced by ventricular pacing (60, 90, 120, 150min −1 ). All images were recorded on a high-speed video system every 4ms. The closing time lag (CTL) between the valve leaflets was calculated and compared between the two groups. Results: In Group H, the lower valve leaflet tended to open incompletely and close earlier than the upper leaflet. The calculated CTL was 303±60ms, 65±48ms, 40±9ms, and 40±26ms at pacing HRs of 60, 90, 120, and 150min −1 , respectively. In contrast to Group H, there was little difference in CTL between the right and left leaflets in Group V. The calculated CTL was 9±12ms, 11±10ms, 1±3ms, and 6±7ms at pacing HRs of 60, 90, 120, and 150min −1 , respectively. There were significant differences in CLT between the two groups at each ventricular pacing rate (P<0.01). Conclusions: Orientation of an implanted bileaflet valve in the tricuspid position significantly influenced leaflet motion. In a horizontal orientation, the lower valve leaflet opened incompletely and closed earlier than the upper leaflet. These results suggest that the gravity might affect leaflet motion and that bileaflet mechanical valves should be implanted vertically in TVR to prevent abnormal leaflet motion and thrombus formation.