Sequential hemodynamic studies were performed in 13 patients having aortic valve replacement with glutaraldehyde-stabilized pericardial xenografts. The investigation done preoperatively was repeated twice in all patients at mean intervals of 9.9 and 42.2 months following valve replacement. Annulus diameters of the valves were 19, 23, and 25 mm. Between the preoperative and the first postoperative investigation there was a significant increase in cardiac index (p less than 0.05) and a significant reduction in both pulmonary wedge and left ventricular end-diastolic pressures (p less than 0.01). At the first postoperative study the mean ejection systolic gradient across the three sizes of aortic pericardial xenograft was 7, 15.7, and 8.4 mm Hg at rest and 10, 19.6, and 13.7 mm Hg on exercise, respectively. The corresponding surface areas of the three valve sizes were 1.1, 1.5, and 1.62 cm2 at rest and 1.4, 2.1, and 2.03 cm2 on exercise. There was no significant change in any of these measurements between the first and the second postoperative study. The results have shown considerable postoperative hemodynamic improvement, insignificant transvalvular pressure gradients, and maintenance of the functional integrity of the pericardial xenograft up to 56 months following valve insertion.