AbstractIntimal thickening in vein grafts is one of the major factors leading to graft failure. Studies have suggested that delayed re‐endothelialization and an enhanced permeability to proliferative factors (macromolecules) in the blood are associated with the development of intimal thickening in these vessels. The aim of this study was to determine the pattern of re‐endothelialization of vein grafts and the permeability of the new endothelium to a large molecule. Iliolumbar vein to iliac artery grafts 5 mm long were inserted microsurgically into 32 male albino Wistar rats. Graft re‐endothelialization and permeability, at times ranging from 2 to 140 days after grafting, were studied by scanning electron microscopy and vital staining with Evans Blue dye, which binds to plasma albumin and was used as a model of a large molecule. Re‐endothelialization of the graft had commenced by 2 days after grafting and in most animals was completed by 15 days. Some grafts exhibited local areas devoid of endothelium up to 4 weeks after grafting. The mechanism of re‐endothelialization involved the migration of a sheet of endothelium across the anastomosis from the adjacent artery. This sheet subsequently linked up with islands of cells which developed in the centre of the graft by 5 days. Evans Blue dye was detected in all areas where endothelium had been removed by the grafting operation. Permeability of the graft to the dye was seen during the 2–4 week period after grafting, a time at which previous studies have shown that intimal thickening develops in this vein grafting model. Thus, delayed re‐endothelialization and prolonged permeability to a large molecule (albumin) does occur in some experimental vein grafts. This may be the basis for intimal thickening which is known to occur i