Objective: Small-sized prosthetic valves used in aortic valve position usually cause a high transvalvular gradient. This situation, especially in people with a high body surface area, causes a patient-prosthetic valve mismatch. In this study, hemodynamic performances of the two new generation, bileaflet, small size mechanical valves were evaluated during rest and stress conditions. Methods: Eighteen patients with implanted 21-mm St. Jude HP (11 cases) and Sorin Bicarbon (7 cases) prosthetic valves in aortic position were evaluated at rest and during dobutamine infusion, 16.3±5.5 months after the operation. Dobutamine infusion was started at a dose of 5 mg/kg/min. The infusion rate was increased every 15 minutes up to 10-20-30 mg/kg/min. Maximum gradient, mean gradient, effective valve area (EVA), effective valve area index (EVAI) and performance index (PI) were calculated during rest and maximum dobutamine infusion rate. Results: When the rest and dobutamine infusion measurements were compared, the maximum and mean gradients were found to be increased moderately, but EVA, EVAI, PI values did not change with increasing heart rates. For St. Jude valve; maximum gradient increased from 30.5±6.5 mmHg to 74.7±33.6 mmHg (p=0.03) and mean gradient increased from 17.9±3.8 to 40.8±23.8 mmHg (p=0.03). For Sorin Bicarbon valve; maximum gradient increased from 31.7±13.3 mmHg to 52.0±11.2 mmHg (p=0.01), mean gradient increased from 16.1±6.4 mmHg to 28.8±1.0 mmHg (p=0.01). The difference was not significant between the two valves with respect to measured parameters during rest and maximum dobutamine infusion. Conclusion: According to our findings, 21-mm Sorin Bicarbon and St. Jude HP mechanical bileaflet valves have good hemodynamic performance during exercise and these types of valves seem to be appropriate for patients with small aortic annulus.