The products of univalent reduction of oxygen, superoxide anion radical, hydrogen peroxide, and the hydroxyl radical, are capable of causing cellular damage and death. They are, therefore, logical candidates as mediators of vascular and parenchymal injury in the central nervous system (CNS). This paper reviews the sources of oxygen radicals in the CNS, their effects on cerebral vessels and on brain and spinal cord parenchyma, and the evidence which implicates oxygen radicals in various pathological conditions of the CNS.