Likewise, studies using a marker of O2 - production, hydroethidine (HEt), have yielded a similar pattern of increased O2 - production in the cortex of male mice and ovariectomized female rats within 1-3h hrs of permanent cerebral ischemia (1, 65-66). In addition, as shown in Figure 1a, work by our laboratory has shown that O2 - production increases rapidly in the hippocampal CA1 region following GCI in both male and female rats, with an elevation occurring as early as 30 min after reperfusion and peak levels observed at 3h after reperfusion (42, 69). As also shown in Figure 1a, E2 treatment strongly attenuated the elevation of O2- levels in the hippocampal CA1 region following cerebral ischemia, which correlated with its neuroprotective effect (42). Further studies showed that the E2 attenuation of O2- levels was associated with a dramatic attenuation of oxidative stress damage in the hippocampal CA1 region at 24h after cerebral ischemia, as determined by measurement of oxidative damage markers for lipid peroxidation (4-HNE) and DNA damage (8-OHdG) (Figure 1b-c) (42). A similar E2 suppression of O2- production was demonstrated in the cerebral cortex following FCI (1). In the next section, we will discuss how E2 may regulate ROS generation in cerebral ischemia with a particular focus on an emerging key enzyme for O2- production, Nicotinamide Adenine Dinucleotide Phosphate (NADPH) oxidase.