Transient global cerebral ischemia resulting from cardiac arrest is known to cause selective death in vulnerable neurons, including hippocampal CA₁ pyramidal neurons. It is postulated that oxygen radicals, superoxide in particular, are involved in cell death processes. To test this hypothesis, we first used in situ imaging of superoxide radical distribution by hydroethidine oxidation in vulnerable neurons. We then generated SOD1 transgenic (Tg) rats with a five-fold increase in copper zinc superoxide dismutase activity. The Tg rats and their non-Tg wild-type littermates were subjected to 10 min of global ischemia followed by 1 and 3 d of reperfusion. Neuronal damage, as assessed by cresyl violet staining and DNA fragmentation analysis, was significantly reduced in the hippocampal CA₁ region, cortex, striatum, and thalamus in SOD1 Tg rats at 3 d, as compared with the non-Tg littermates. There were no changes in the hippocampal CA₃subregion and dentate gyrus, resistant areas in both SOD1 Tg and non-Tg rats. Quantitative analysis of the damaged CA₁ subregion showed marked neuroprotection against transient global cerebral ischemia in SOD1 Tg rats. These results suggest that superoxide radicals play a role in the delayed ischemic death of hippocampal CA₁ neurons. Our data also indicate that SOD1 Tg rats are useful tools for studying the role of oxygen radicals in the pathogenesis of neuronal death after transient global cerebral ischemia.