Objective— The chemokine fractalkine activates platelets and induces leukocyte adhesion to the endothelium. Expression of fractalkine and its receptor, CX 3 CR1, is elevated in coronary artery disease. We assessed the effects of fractalkine on vascular function in isolated rat aorta. Methods and Results— CX 3 CR1 expression was demonstrated in rat aortic endothelial and smooth muscle cells by immunohistochemistry, Western blot, and polymerase chain reaction (PCR). Fractalkine (up to 1 μg/mL) did not directly induce contractile or relaxant responses when applied to rat aortic rings in organ baths. Short-term incubation with fractalkine (1 μg/mL) for 5 minutes did not affect vascular reactivity. Pretreatment of isolated rat aortic rings with fractalkine for 2 hours impaired acetylcholine-induced nitric oxide (NO)-mediated relaxation after preconstriction with phenylephrine in a concentration-dependent manner. The concentration response to the NO donor DEA-NONOate was significantly shifted to the right. The radical scavenger tiron normalized the attenuated acetylcholine-induced relaxation after fractalkine incubation. Aortic superoxide formation was enhanced by fractalkine, which was inhibited by diphenyleneiodonium but not by inhibitors of xanthine oxidase or NO synthase. Conclusion— In addition to its role as a chemokine and adhesion molecule, fractalkine induces vascular dysfunction by stimulating vascular reactive oxygen species resulting in reduced NO bioavailability.