Interpositionally grafted arteries and veins were expanded with a 20cc tissue expanders in 50 Sprague-Dawley rats. The grafts were done on both hind legs, one side was expanded and the remaining side was used as control. The average gain in length of expanded grafted arteries and veins was over 4 and 6 times that of the controls respectively. The differences in the patency rates between expanded and control grafts were not statistically significant. Histologic examination revealed that there were no changes in the areas of the media and lengths of the inner elastic laminae of the expanded arterial grafts. In both expanded and control vein grafts, marked intimal thickening was noticed, although these changes were not statistically significant. Expansion of grafted vessels can be safely carried out without loss of vessel patency.