Oxygen free radicals have been widely implicated in the pathogenesis of brain injury due to ischemia followed by reperfusion. The success of making transgenic animals overexpressing human CuZn-superoxide dismutase (CuZn-SOD) in brain cells allows researchers to discern the specific role of superoxide radicals in reperfusion injury after focal ischemia. It has been shown that increased brain levels of CuZn-SOD in transgenic mice protect neurons from ischemia/reperfusion injury. However, overexpression of CuZn-SOD does not provide neuronal protection in permanent focal ischemia in mice, when compared with non-transgenic mouse littermates. It is proposed that molecular genetic approaches of modifying antioxidant levels in the brain offer a unique tool for studying oxidative mechanisms in focal cerebral ischemia.