To better comprehend the role of the endothelial lining in occurrence and development of intimal thickening in arterially implanted autogenous vein grafts, two models of canine femoral arteries were prepared. In the Group I model, the implanted autogenous vein graft was kept under a normal flow condition for 2 to 4 weeks after implantation, then was exposed to an abnormal flow (poor run-off). In case of a 3- to 4-week normal flow, intimal thickening was practically nil. Scanning electron microscopic studies showed that this 3- to 4-week period corresponded to that of re-endothelialization of the autogenous vein grafts. Immunohistochemical studies of fibrinogen distribution in the autogenous vein graft wall in the Group II model revealed that the permeation of fibrinogen was enhanced in case of an abnormal flow condition for about 2 weeks after the implantation. We interpret these observations to mean that intimal thickening was induced by an accelerated permeability in the presence of abnormal flow conditions until full re-endothelialization after the implantation.