By attaching appropriate measuring devices to the wall of an intact [dog] aortic root at the level of leaflet coaptation, a 16% diameter change was measured during each cardiac cycle. The dimensional changes observed can by themselves explain aortic valve function and obviate the postulation that the leaflets shorten and lengthen during each cardiac cycle. The tissue composition of the aortic root and leaflets is more compatible with this theory than with other postulations. Such a dynamic aorta root may explain the longevity of the actual aortic leaflets, in that leaflet fatigue stress is minimized by changes in aortic root dimension.