Despite improvements in the care of patients with aneurysmal subarachnoid hemorrhage, delayed cerebral vasospasm remains a major cause of morbidity and death. There is now evidence that a decrease in the local availability of nitric oxide (NO) plays a role in delayed cerebral vasospasm. We evaluated a controlled-release polymer containing the NO donor (Z)-1-[2-(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate (DETA/NO) for the treatment of chronic posthemorrhagic vasospasm in the rat femoral artery model. The release kinetics of ethylene/vinyl acetate copolymers loaded with 20% (w/w) DETA/NO were determined in vitro. Chronic vasospasm was induced in the left femoral artery of adult male Fischer 344 rats (n = 35) by exposure to autologous blood. At 1, 3, or 7 days after blood exposure, either a 5-mg polymer loaded with 20% (w/w) DETA/NO or an empty 5-mg polymer was placed in the periadventitial space next to the left femoral artery. At the same time, an empty 5-mg polymer was placed next to the right femoral artery. On the 8th day after blood exposure (at the peak of vasospasm in this model), rats were transcardially perfused with 4% paraformaldehyde, and the left and right femoral arteries were removed for histological processing and morphometric analyses. Vasospasm was expressed as the percent lumen patency of the treated left artery, compared with the control right artery. The in vitro release kinetics demonstrated that the 20% DETA/NO-loaded polymers released up to 15% of their total drug load during a 9-day period. DETA/NO treatments initiated at 1, 3, or 7 days after blood deposition all significantly inhibited vasospasm, compared with control values (94.6 +/- 7.2% versus 67.6 +/- 5.8%, 104.6 +/- 5.5% versus 64.9 +/- 1.7%, and 102.4 +/- 5.1% versus 73.6 +/- 1.4%, respectively; mean +/- standard error of the mean percent lumen patency; P < 0.001). No adverse effects of treatment were observed. The diazeniumdiolate NO donor DETA/NO can be effectively released from ethylene/vinyl acetate polymers. Administration of DETA/NO into the periadventitial space can prevent the development of chronic posthemorrhagic vasospasm in the rat femoral artery and can reverse established vasospasm. No adverse effects of DETA/NO were observed in this model.