Studies of the structure of blood vessels have commonly found that cells lining the vessels are lengthened in the direction of blood flow, presumably as an effect of shear stress. A study of aortic valves has, however, shown that endothelial cells covering both sides of aortic valve leaflets are arranged across the direction of flow. Aortic valve leaflets taken from seven adult mongrel dogs and fixed either open or closed with glutaraldehyde were studied with a scanning electron microscope after critical point drying. On both open and closed leaflets the endothelial cells tended to be aligned circumferentially with the free edge of the leaflet. The circumferential pattern was particularly unexpected on the leaflet surface facing the left ventricle because this surface could be expected to receive the full effect of shear stress from systolic blood flow. The finding suggested that surface cellular alignment on leaflets is determined by some force other than shear stress. The force apparently responsible for organising the collagenous layers in the leaflet, which are best adapted to bear stress, is diastolic blood pressure. Since the endothelium follows the same pattern of alignment as the layers of collagen it seems reasonable to conclude that endothelial orientation is also a response ultimately to pressure stress. The arrangement of the endothelium then serves as a readily observed indicator of those functional stresses that dominate and organise leaflet structure.