The reported systolic enlargement of the aortic root in the adult, and the resultant sharing of leaflet stress with the aortic wall, could explain the longevity of aortic leaflets. Previous studies of the aortic root have been confined to the excised specimen; this study, however, was undertaken to determine the in vivo dynamic relationship between leaflet motion and aortic root expansion. In each of nine adult dogs, we attached four Dacron sutures to appropriate parts of the leaflet and commissures to connect them to linear displacement transducers during right heart bypass. Changes in leaflet length during the cardiac cycle were measured. Diastolic leaflet lengths were measured from silicone rubber casts made at 80 mm. Hg. Histologic evaluation was performed to provide a structural basis for understanding the in vivo measurements of changes in leaflet length. The results indicate that, during the cardiac cycle, changes in leaflet length are less than 2 percent. This observation, which is supported histologically by the orientation of collagenous fibers, suggests that the leaflets resist repeated lengthening and shortening. The date support the hypothesis that the entire aortic root plays a critical role in the opening and closing of the normal aortic valve and that the leaflets sustain less stress than has been postulated previously.