Background: Statins have recently been shown to exert neuronal protection in ischemic stroke. Reactive oxygen species, specifically superoxide formed during the early phase of reperfusion, augment neuronal injury. NADPH oxidase is a key enzyme for superoxide production. The present study tested the hypothesis that atorvastatin protects against cerebral infarction via inhibition of NADPH oxidase-derived superoxide in transient focal ischemia. Methods: Transient focal ischemia was created in halothane-anesthetized adult male Sprague-Dawley rats (250-300 g) by middle cerebral artery occlusion (MCAO). Atorvastatin (Lipitor^TM, 10 mg/kg) was administrated subcutaneously 3 times before MCAO. Infarct volume was measured by triphenyltetrazolium chloride staining. NADPH oxidase enzymatic activity and superoxide levels were quantified in both the ischemic core and penumbral regions by lucigenin (5 µM)-enhanced chemiluminescence. The expression of NADPH oxidase membrane subunit gp91^phox and membrane-translocated subunit p47^phox and small GTPase Rac-1 were determined by Western blot analyses. Results: NADPH oxidase activity and superoxide levels increased following reperfusion and peaked within 2 hours of reperfusion in the penumbra, but not in the ischemic core in MCAO rats. Atorvastatin pretreatment prevented this increases, blunted the expression of membrane subunit gp91phox and prevented the translocation of cytoplasmic subunit p47^phox to the membrane in the penumbra 2 hours after reperfusion. Consequently, cerebral infarct volume was significantly reduced in atorvastatin-treated compared to non-treated MCAO rats 24 hours after reperfusion. Conclusions: These results indicate that atorvastatin protects against cerebral infarction via inhibition of NADPH oxidase-derived superoxide in transient focal ischemia.