There is now a considerable amount of evidence in the literature implicating oxygen-derived free radicals in the vascular permeability changes associated with intestinal ischemia. To directly assess the effects of oxygen radicals on vascular permeability, hypoxanthine xanthine oxidase, an enzyme-substrate system known to generate oxygen free radicals, was infused into the arterial supply of autoperfused segments of cat ileum. The osmotic reflection coefficient (σ(d)) of intestinal capillaries to total plasma proteins was estimated from the steady-state relationship between lymph-to-plasma total protein concentration ratio and lymph flow. Intra-arterial infusion of hypoxanthine-xanthine oxidase reduced σ(d) from a control value to 0.92 to 0.66, indicating an increased vascular permeability. This increase in vascular permeability was significantly attenuated by the addition of superoxide dismutaase (σ(d) = 0.86), a specific scavenger of superoxide anion (O2-), or dimethylsulfoxide (σ(d) = 0.83), the hydroxyl radical (OH·) scavenger, to the infusate. The results of this study indicate that oxygen-derived free radicals, generated by the reaction of hypoxanthine with xanthine oxidase, increase intestinal vascular permeability to an extent comparable with that observed in preparations subjected to 1 h of ischemia.