To determine how hemodynamic forces influence the permeability of endothelial cells, we observed the composition of junction complexes of the endothelial cells at the flow dividers of bifurcations of the aorta in normolipidemic and hyperlipidemic rabbits by a freeze-fractured replica method. We found that the leading edge of the brachiocephalic flow dividers exposed to high shear stress in a laminar fashion was nonsudanophilic and was covered by long fusiform endothelial cells predominantly showing continuous tight junctions, rather than discontinuous forms, and smooth, regular gap junctions. In contrast, the "hip" of the brachiocephalic flow dividers, a region of relatively low shear stress, was sudanophilic, and was covered by ellipsoidal cells. The tight junctions were primarily discontinuous and the gap junctions had an irregular shape. These findings suggest that exposure of the arterial wall to a relatively low wall shear stress, rather than to high shear stress, may functionally activate the endothelial cells, increasing intercellular permeability. These events may increase the vulnerability of these regions of the vessel to atherosclerosis.