F-actin microfilament reorganization in response to alterations in shear stress has not been experimentally tested in vivo. In the current study, we analyzed changes in F-actin distribution in endothelial cells around the site of a coarctation performed in the midabdominal aorta of rabbits. The coarctation caused a 60% decrease in luminal diameter and produced three distinct zones: 1) a high shear region immediately upstream of the coarct (Zone I); 2) a region of low, fluctuating shear immediately downstream of the coarct (Zone II); and 3) an annular vortex characterized by high shear extending 0.5 to 3 mm downstream of the coarct (Zone III). Endothelial cells of control abdominal aortas were ellipsoid in shape and aligned in the direction of blood flow. They displayed a prominent circumferential band of microfilaments and short, thin stress fibers. Near coarctations, cells of Zone I were much more elongate, and stress fibers were markedly thicker and longer than in control abdominal aortas. In Zone II, cells were polygonal in shape and showed a prominent peripheral band of microfilaments and central stress fibers. Zone III cells were similar in shape to control abdominal aortic endothelial cells but showed very striking central stress fibers. These findings indicate that in vivo F-actin microfilament distribution can be modulated by experimentally altering flow conditions. F-actin redistribution in response to elevated shear stresses may increase cell-substrate adhesion and thus maintain endothelial integrity.