Apocynin neuroprotection against cerebral ischemia was associated with reduced levels of apoptotic factors and markers, such as Bax, Bcl-2 and TUNEL staining (83), suggesting that NADPH oxidase activation plays a key role in the induction of apoptosis following cerebral ischemia. Additional work by our laboratory showed that administration of a specific competitive NOX2 inhibitor, gp91ds-tat, significantly attenuated elevation of NADPH oxidase activity and O2- levels in the hippocampal CA1 region following GCI, and was strongly neuroprotective (42). This suggests that NOX2 NADPH oxidase plays a significant role in the elevation of O2- and resultant neuronal damage in the hippocampus following cerebral ischemia. Further work by our laboratory and others demonstrated that NOX2 is predominantly localized in neurons in the hippocampus following cerebral ischemia (42), but also appears in microglia at later time-points after cerebral ischemia (84). In situ O2- determination using the hydroethidine method also revealed O2- elevation in neurons, with some occurring in microglia/ macrophages, and little in endothelial cells in the cortex and hippocampus at early timepoints after cerebral ischemia (42, 85). There is also some evidence that NOX2-derived O2- from circulating lymphocytes that infiltrate the infract area may also contribute to O2- elevation at the infarct site (86).