Experimental animal studies have demonstrated that oxidative stress plays an essential role during ischemic stroke. In addition to oxidizing macromolecules leading to cell injury, oxidants are also involved in cell death/survival signal pathways and cause mitochondrial dysfunction. Nuclear factor erythroid 2-related factor 2 (Nrf2) represents one of the major regulators implicated in the endogenous defense system against oxidative stress. We have studied the expression and activation status of Nrf2 under stroke-like conditions using the temporary middle cerebral artery occlusion rat model. Inactive Nrf2 is proteasomal degraded within minutes but stabilized during activation. We analyzed Nrf2 activation and the resulting accumulation in post-ischemic rat brain cells using double immunofluorescence staining with antibodies directed against Nrf2 and cell type-specific markers. The core infarct region showed no obvious positive staining signal for Nrf2 24 h after the initiation of artery occlusion. However, Nrf2 immunoreactivity was detectable in the ipsilateral penumbra where microglia, astrocytes, and neurons contained Nrf2. Interestingly, Nrf2 was also significantly upregulated in neurons but not in other cell types of the unaffected contralateral site. These results provide strong evidence that Nrf2 is involved in acute stroke-dependent neurodegeneration in the penumbra but not core region and indicate the presence of a systemic Nrf2 activator independent from oxidative stress.