This study describes the effects of CO2 laser radiation on the histology of the normal rabbit arterial wall, using models that simulate laser angioplasty and anastomosis. Rabbit arteries were exposed to laser treatments similar to those used clinically; 40, 0.5 sec pulses of 40-60 mW, CO2 continuous wavelength laser, or a 1/2-circumferential laser anastomosis with a 60-80 mW continuous pulse. Aneurysms developed in 8 of 22 femoral, 1 of 22 carotid, and no controls at 12 week. There were small breaks in the internal elastic lamina with atrophy, loss of muscularis, "packing" of the elastica, thinning of the muscularis at the damage site, and enlargement of the arterial diameter. Aneurysms developed in one femoral and no carotid anastomosed artery. Laser anastomoses demonstrated more muscle damage and loss, with extensive scarring and a wider area of elastic loss than the controls. The intima was reestablished with focal reduplication of the internal elastic lamina. There were no histologic differences between the arteries which developed aneurysms and those which did not in either series. These results suggest that low power laser damage of the arterial wall consists mainly of destruction of the muscularis propria, with minimal damage to the elastica.