The Medtronic-Hall aortic valve prosthesis has been studied in order to evaluate its efficacy to respond to moderate (50 Watts) and heavy (100 Watts) physical exercise. A marked increase in cardiac index from 2.86 +/- 0.45 l/min/m2 BSA at rest to 6.80 +/- 0.51 l/min/m2 BSA at the heaviest work load was well-tolerated. Peak pressure gradient rose from 7.9 +/- 8.7 mmHg at rest to 23.0 +/- 16.3 mmHg during the heaviest physical exercise. Utilization of the available valve orifice area was acceptably good. The discharge coefficient, which is the ratio between the effective orifice area and the internal stent orifice area, was 0.69 +/- 0.17 at rest. The performance index, which is the ratio between the effective orifice area and the annulus area, was 0.44 +/- 0.11 at rest. Both indices tended to increase during maximal exercise, possibly due to a more favorable position of the valve in relation to flow pattern.