Objectives. We tested the hypothesis that dismutation of superoxide anion increases endogenous levels of nitric oxide, resulting in inhibition of cyclic variations in blood flow in arteries that are injured and stenotic. Background. Platelet adhesion and aggregation leading to cyclic flow variations might result, in part, from generation of superoxide anion that can deplete endogenously produced nitric oxide. Methods. Spontaneous cyclic flow variations, monitored with a proximal Doppler probe, were induced in the carotid artery of anesthetized rabbits by clamping the vessel with forceps and placing a high grade stenosis at the site of injury. Bovine copper/zinc superoxide dismutase (12 mg/kg body weight, n = 5), a synthetic low molecular weight mimetic (12 mg/kg, n = 8) or buffer vehicle (n = 8) was administered intravenously as divided boluses over 45 min, and the frequency of cyclic flow variations was monitored for 4 h. Results. Cyclic flow variations remained stable for 4 h in vehicle-treated animals (15 ± 1 [mean ± SEM]/30 min at baseline and 16 ± 1/30 min after 4 h, n = 8) but exhibited a marked and persistent reduction in animals given copper/zinc superoxide dismutase (from 14 ± 1/30 min at baseline to 4 ± 1/30 min after 4 h) or the mimetic (from 15 ± 1/30 min at baseline to 3 ± 1/30 min after 4 h, p < 0.005). They were restored in three of four mimetic-treated animals during infusion of NG-monomethyl-l-arginine (100 mg/kg), an inhibitor of nitric oxide production. In addition, levels of cyclic guanosine 5′-monophosphate in platelets were elevated after administration of the mimetic (from 2.4 ± 0.5 fmol/106 platelets at baseline to 4.9 ± 0.6 fmol/106 platelets 45 min after the mimetic, p < 0.03, n = 6), whereas mean arterial blood pressure was decreased and flow velocity in the carotid artery was increased consistent with mediation of the effect on cyclic flow variations by increased endogenous nitric oxide. Conclusions. Dismutation of superoxide anion appears to attenuate platelet thrombus formation at a site of vessel injury by potentiation of endogenously produced nitric oxide. This approach may have utility to inhibit platelet-rich thrombosis in injured and stenotic arteries where production of superoxide anion is increased.