The objective of this study was to determine whether agents that either scavenge or inhibit the production of oxygen radicals can alter the adhesive interactions between leukocytes and venular endothelium elicited by ischemia-reperfusion. Cat mesenteric and intestinal blood flows were reduced to 20% of baseline for 1 hr, followed by 1 hr of reperfusion. Sixty minutes after reperfusion, red blood cell velocity (Vr), leukocyte rolling velocity (Vw), and the number of adherent leukocytes were measured in mesenteric venules. Then, either manganese-superoxide dismutase (Mn-SOD), catalase, desferrioxamine, or oxypurinol was administered intravascularly. Ten minutes later, repeat measurements were obtained and compared with pretreatment values. Catalase, Mn-SOD, and oxypurinol significantly attenuated neutrophil adherence while neither inactivated-catalase nor desferrioxamine altered the reperfusion-induced leukocyte adhesion. The ratio of Vw to erythrocyte velocity, an index of the fracture stress between rolling leukocytes and venular endothelium, was not altered by any of the agents studied. These results and data in the literature indicate that many of the agents that are commonly used to either scavenge or inhibit the production of oxygen radicals in postischemic tissues exert a significant inhibitory influence on leukocyte adhesion to microvascular endothelium in vivo. Our results are also consistent with the view that xanthine oxidase-derived oxidants contribute to the leukocyte-endothelial cell adhesive interactions associated with reperfusion of ischemic tissues.