Lasers have used safely to and in the resection of highly vascular tumors or arteriovenous malformation. As far as blood vessel coagulation, arteries up to 2mm in diameter and veins up to 3mm in diameter can be reliably sealed with the Nd-YAG laser, and lesser size of vessels can be coagulated with the CO2 laser. Another advantage of laser is to be a useful tool for small vessel repair or anastomosis without microsuture technique. The aim of this experiment was to investigate intensity and duration of exposure with the CO2 laser for vessel coagulation and repair. 36 rats weighing 200 to 250 gm, were used in this study. For vessel coagulation of extracranial arteries and veins, power density between 10 and 50 watts/cm2(W) was applied with handpiece, and for vessel repair or anastomosis 2 W was applied with micromanipulator. The results obtained were as follows : 1) The artery was perforated immediately over 10 W with 0.5 seconds of exposure of smaller focal beam. 2) The artery was constricted in its diameter in defocused beam with 30-40 W, and perforated with 50 W. 3) The lower limit of venous coagulation was 15 W, 0.5 sec. of exposure and 2 mm of focal beam diameter. 4) The lower limit or arterial coagulation was 10 W, 5 sec. of exposure, 4 mm of focal beam diameter, and exposure time was reduced to 3 sec. at 20 W. 5) The repair or anastomosis of artery was accomplished with 2 W, 0.05 sec. of exposure of 0.2 mm of focused beam by 6 times of delivery, and the delivery times could be successfully reduced to half under the overlying thin blood clot film on the anastomosis site of the artery. 6) In the histological examination, the entire layer of coagulated vessel wall on the laser delivered side disclosed marked constriction and destruction. On the anastomosis site, re-endothelialization began from 3 days, and completed by 2 weeks after operation.