An in vitro comparison of the hydrodynamic characteristics of the Edwards-Duromedics (DM) and St Jude (SJ) bi-leaflet aortic valve prostheses is presented. Aortic valves 27 mm in diameter were mounted in a pulse duplicator simulating physiological pulsatile flow using a glycerol solution as the blood analogue fluid. Mean trans-valvular pressure difference (TPD) in systole, the per cent regurgitation (PCR) and the projected valve orifice area (VOA) were compared at a range of flow rates and heart rates to reflect the functioning of the valve under resting as well as exercise conditions. Our results showed that the TDP for DM (0.99 +/- 0.28 kPa) valve was slightly larger than the SJ (0.81 +/- 0.19 kPa) valve (mean difference of 0.18 +/- 0.26 kPa, P less than 0.05). However, the PCR for the DM (13.6 +/- 7.8) valve was smaller than the SJ (17.9 +/- 9.0) valve (mean difference of 4.3 +/- 4.2%, P less than 0.01). Moreover, VOA for the DM (2.51 +/- 0.10 cm2) valve was smaller than the SJ (3.13 +/- 0.04 cm2) valve (mean difference of 0.63 +/- 0.10 cm3, P less than 0.001).