The authors probed the vascular endothelial cell cytoskeleton in strains of pigeons that are atherosclerosis-susceptible and disease-resistant, namely, the White Carneau and Show Racer pigeons. Endothelial cell actin and myosin were localized with the use of affinity-purified antibodies in conjunction with indirect immunofluorescence microscopy. The endothelial cell cytoskeleton was characterized in a site-specific and time-dependent manner by examination of arterial segments from each strain of pigeons. Anti-actin and anti-myosin fluorescence staining patterns of endothelial cells lining the ascending aorta, aortic arch, and thoracic aorta from the White Carneau and Show Racer pigeons sacrificed at 1 and 12 months of age were compared and analyzed. In the Show Racer, irrespective of arterial site or chronologic age, endothelial cell cytoskeletal organization is similar. Actin and myosin fluorescence is brightest at the cortex, where endothelial cells meet their neighbors. There is also an amorphous (diffuse) fluorescence throughout the cytoplasm. In addition to the diffuse and cortical cytoskeletal fluorescence in the endothelial cells of the Show Racers, the White Carneau also possess a unique cytoskeletal array of linear fluorescence, ie, the endothelial cell ridge. At 1 month of age, anti-actin staining of endothelial cell ridges averages 28.5 mu in length in the ascending aorta, 28.0 mu in the aortic arch, and 40.0 mu in the thoracic aorta. At the same time, anti-myosin fluorescence extends past both ends of the anti-actin-stained endothelial cell ridge fluorescence. In the ascending aorta, anti-myosin labeling of endothelial cell ridges is 3.5 times longer than anti-actin staining. This staining is absent in the aortic arch, whereas the thoracic aorta possesses endothelial cell ridges that extend over the entire length of the vessel segment. At 12 months of age, actin-stained endothelial cell ridges increase 1.6- and 1.4-fold in the ascending aorta and aortic arch, respectively. The thoracic aorta possesses endothelial cell ridges that cover its entire length. At 12 months of age, the length of myosin-stained endothelial cell ridges does not increase in the ascending or thoracic aorta. In contrast, the aortic arch expresses endothelial cell ridges that exceed 150 mu in length. It is proposed that the endothelial cell ridge assembles from cytoskeletal components as a focal endothelial cell response to injury, perhaps promoting endothelial cell adhesion to the underlying basal lamina through a transmembrane linkage.