Carbon dioxide laser which is used widely in neurosurgery has less traumatic effect on the tissue beside the lesion than other types of laser. There has been little study on the changes to the vascular tissue, following laser application. We performed a study on 4 goups of dogs; comprising craniectomy only, craniectomy and thyrotropin releasing hormone (TRH), craniectomy and laser, and craniectomy, laser and TRH. We applied CO2 laser on the angular artery of two groups of dogs. To one group we infused TRH for 15 days following the laser application and to the other group we did not. We investigated the ultrastructural and biochemical changes and the effect of TRH on the tissue 15 days following laser application. In our study, we observed an increase in superoxide dismutase activity and a decrease in ATP-ase activities in the degenerated tissue on which CO2 laser was applied. TRH partially prevented the vascular degeneration of CO2 laser but did not significantly change superoxide dismutase and Adenosine-5'-triphosphatase activities.