Purpose : Histopathologic observation was performed in order to determine which type of suture material is superior in microvascular anastomoses. Materials & Methods : The interrupted end to end anastomosis of the transected carotid arteries of 105 Sprague-Dawley rats, weighing 180 to 200g, were performed using 9-0 polypropylene ($Prolene^{TM}$, Ethicon, U.K.), 9-0 polyglactin 910 monofilament($Vicryl^{TM}$, Ethicon, U.K.), and 9-0 polyamide($Ethilon^{TM}$, Ethicon, U.K.) under intramuscular Ketamine and Xylazine anesthesia(5mg/100g). In all cases, 10 to 12 sutures were placed to complete the anastomoses. The specimens were obtained at 1, 2, 3 days, and 1, 2, 4 and 6 weeks after the surgery and prepared with H&E and Van-Gieson stains and investigated the histologic changes in anastomotic sites under light microscope. The histologic changes we were concerned about were followings- thrombus formation, intimal edema, infiltration of inflammatory cells in media and adventitia, proliferation of endothelial cells and subintimal hyperplasia. Results : 1. All of the anastomosed arteries were patent when they were exposed for examination. 2. Thrombus formation and intimal edema were most severe in Vicryl group, followed by Ethilon, Prolene group in order. 3. The inflammatory cells infiltrated to the media and the adventitia most severely in Ethilon group, followed by Vicryl, Prolene group in order. 4. There was little difference in proliferation of endothelial cells in each group. 5. Subintimal hyperplasia was greater in Vicryl group than the others, but there was no significant difference between the Prolene and Ethilon groups. Conclusions : On the basis of these observations, we could conclude that Prolene may be the better suture material for microvascular anastomoses regarding the tissue responses than Ethilon and Vicryl.