The sequential univalent reduction of oxygen generates superoxide, hydrogen peroxide, and hydroxyl radical. The generation of hydroxyl radical is dependent on catalysis by ferrous iron. In addition, superoxide and nitric oxide produce peroxynitrite, which spontaneously generates hydroxyl radical independently of iron-mediated catalysis. These agents have a variety of cellular actions, which render them suitable candidates as mediators of tissue destruction and cellular death. In the intact brain, superoxide and its derivatives cause vasodilation, mediated by opening of potassium channels, altered vascular reactivity, breakdown of the blood-brain barrier, and focal destructive endothelial lesions. These abnormalities are also seen in early reperfusion following brain ischemia. During reperfusion there is a marked transient increase in superoxide production. Vasodilation, abnormal vascular reactivity, and blood-brain barrier breakdown are inhibited by eliminating superoxide. Superoxide production during reperfusion may be initiated by glutamate via activation of α-amino-3-hydroxy-5-methylisoxasolepropionic acid (AMPA) receptors. These experimental findings have important implications for human cerebral ischemia. Agents directed at eliminating oxygen radicals must be administered before or in the early stages of reperfusion following ischemia. The therapeutic window appears to be narrow and limited to, at most, a few hours. The inhibition of AMPA receptors may be a promising approach to inhibit the production of oxygen radicals during ischemia-reperfusion of the brain.