The effects of total hip replacement on the condition of the luminal surface of the jugular and femoral veins and the carotid and femoral arteries were studied in healthy dogs. Four hours after the completion of surgery, most of the blood was removed by whole-body perfusion and the vessels were partially fixed in situ by glutaraldehyde perfusion, harvested, and prepared for scanning electron microscopy. In control dogs (held under anesthesia) the luminal surfaces of the veins (including valves and confluences) and arteries were covered by intact endothelium and were free of adhering blood cells. The arteries from dogs that were operated on were also free of damage. In contrast, the veins from dogs that were operated on showed a series of roughly parallel microtears around 70 to 80 per cent of the confluences of the side branches with the jugular or femoral vein. The basement membrane as well as the endothelial sheet was torn, thereby exposing underlying connective-tissue fibers and smooth muscle cells. The endothelium separated along intercellular junctions. The tears were infiltrated with many leukocytes and platelets and some erythrocytes. Subtle endothelial changes and patchy leukocyte adhesion occurred away from the side branches. The mechanism or mechanisms remain to be elucidated.