The function of a bovine pericardial xenograft valve in the mitral position was studied in 18 patients by using stress Doppler echocardiography after submaximal supine exercise. In 10 patients with normally functioning tissue valve, peak and mean gradients increased from 9.3 +/- 1.3 and 2.7 +/- 0.7 mmHg, respectively at rest, to 14.7 +/- 0.9 and 4.1 +/- 1.0 mmHg at peak exercise. The tissue valve area assessed by the Doppler pressure half-time method increased from 2.69 +/- 0.38 cm2 at rest, to 3.15 +/- 0.38 cm2 at peak exercise (P < 0.05). In 6 patients with mitral regurgitation, peak and mean gradients increased from 17.3 +/- 5.3 and 7.0 +/- 1.8 mmHg, respectively at rest, to 24.3 +/- 5.0 and 9.0 +/- 2.6 mmHg at peak exercise, but there was no significant change in valve area at peak exercise. Patients with severe isolated mitral regurgitation had a Vmax of more than 2 m/s and elevated pressure gradients at peak exercise. But the pressure half-time was relatively unaffected, allowing their differentiation from patients with tissue valvular stenosis. Stress Doppler echocardiography is a simple, noninvasive and reliable method for the evaluation of the mitral tissue valve function which is recommendable for patients with suspected mitral tissue valve dysfunction.