The preventive effects of gene transfer of human copper/zinc superoxide dismutase (Cu/ZnSOD) on the development of cerebral vasospasm after subarachnoid hemorrhage (SAH) were examined using a rat model of SAH. An experimental SAH was produced by injecting autologous arterial blood twice into the cisterna magna, and the changes in the diameter of the middle cerebral artery (MCA) were measured. Rats subjected to SAH exhibited a decreased diameter with an increased wall thickness of MCA that were significantly ameliorated by pretreatment with diphenyleneiodonium (DPI, 10μM), an inhibitor of NAD(P)H oxidase. Furthermore, application of recombinant adenovirus (100μl of 1⁓1010 pfu/ml, intracisternally), which encodes human Cu/ZnSOD, 3 days before SAH prevented the development of SAH-induced vasospasm. Our findings demonstrate that SAH-induced cerebral vasospasm is closely related with NAD(P)H oxidase-derived reactive oxygen species, and these alterations can be prevented by the recombinant adenovirus-mediated transfer of human Cu/ZnSOD gene to the cerebral vasculature.