In order to define the respective roles of the vasa vasorum blood supply and intraluminal arterial blood flow in maintaining the endothelial integrity of in situ vein bypasses, we have carried out two separate but interrelated experiments in a canine model. In vivo studies on eight dogs demonstrated that even in the absence of intraluminal blood flow the vasa vasorum maintained endothelial integrity and also showed that the endothelium was very sensitive to the loss of the vasa vasorum blood supply. In a second group of experiments on 14 in situ vein bypasses we studied the effect of division of the vasa vasorum immediately after arterialization of the bypass. This experiment showed that arterialization of the vein maintained endothelial integrity despite division of the vasa vasorum.