The oxygen radical-producing enzyme xanthine oxidase (XO) can promote neutrophil adherence to endothelium. Recognizing that a balance often exists in inflammatory processes, we sought to determine whether XO initiates antiadherent pathways. We found that bovine pulmonary arterial endothelial cells (EC) exposed to XO released increased amounts of nitrite into the media, reflecting an increased production of nitric oxide (NO). When EC were subjected to shear stress, treatment with XO and/or the NO synthase inhibitor N omega-nitro-L-arginine (L-NNA) increased neutrophil rolling behavior and firm neutrophil adherence to EC in an additive fashion. Both rolling and adherent interactions were abolished by monoclonal antibodies directed against P-selectin. In addition, treatment of EC with XO and/or L-NNA increased both surface expression of P-selectin and release of von Willebrand factor into media. Finally, treatment of EC with the NO donor sodium nitroprusside decreased XO-mediated neutrophil rolling and adherence. We conclude that XO stimulates EC to produce NO and that NO decreases the P-selectin-dependent neutrophil adhesion initiated by XO. Such increases in endogenous NO may constitute an important negative-feedback response to the acute proadhesive effects of XO.