A knowledge of the behavior of the aortic valve sinuses is necessary to the understanding of stress sharing between the sinuses and the leaflets. Radiopaque markers were placed on the sinuses and the leaflets of dogs during cardiopulmonary bypass, and the movement of the markers was studied using fluoroscopy. The center of the sinus moved radially during each cardiac cycle, but in an inconsistent manner. The sinus was under a dual influence: the passive influence of aortic pressure and the active influence of myocardial contraction. The longitudinal curvature of the sinus showed no dimensional change, whereas the radius of the circumferential curvature decreased by 15.7% from systole to diastole. In diastole, the stress in the sinus was 6.1 g/mm2 and was 24.3 g/mm2 circumferentially and 12.1 g/mm2 radially in the leaflet. Histologically, the main stress-bearing component of the leaflet was made up of thick, dense, collagenous fibers oriented circumferentially. These fibers curved into the sinus wall instead of inserting straight into the aortic wall, thereby suggesting that the high stress in the leaflet is shared with the sinus and that continuity of the circumferential stress exists between the leaflet and the sinus. The leaflet does not pull inwardly on the aortic wall. In diastole, the sinus adapts to the new stress conditions in the leaflet by reducing its radius of circumferential curvature. This stress sharing is important for the longevity of the aortic valve.