Object. Hydrogen peroxide (H(2)O(2)) is used as a hemostatic agent in many neurosurgery centers. The authors used a 3% H(2)O(2) solution for final hemostasis after removal of a left insular tumor. Immediately afterward, air bubbles were observed within the lumen of the polar temporal artery. Postoperative MR imaging revealed punctate areas of infarction in the lenticulostriate artery territory. The authors designed an experimental Study to elucidate the mechanism of remote O(2) emboli and reactive O(2) species-related vasoactive responses and thrombus formation.