Platelet-activating factor (PAF), released during inflammatory responses, increases microvascular permeability to fluid and macromolecules. Previous studies in the hamster cheek pouch microcirculation have shown that PAF-induced increases in permeability can be diminished by pretreatment with a nitric oxide synthase inhibitor indicating that nitric oxide is required for PAF to cause leakage, although nitric oxide itself does not cause leakage. We evaluated the hypothesis that PAF stimulates the production of reactive oxygen species (ROS) that then react with nitric oxide to form a new species that signals the increase in vascular permeability. The hamster cheek pouch microcirculation was used to quantify the leakage of FITC-dextran following topical application of PAF. PAF-induced leakage was markedly inhibited (70%) by prior superfusion of the cheek pouch with superoxide dismutase and catalase. Superfusing the cheek pouch with ROS generated by xanthine oxidase and hypoxanthine produced leakage similar to that observed with PAF. Pretreating the cheek pouch with a nitric oxide synthase inhibitor (N<sup>ω</sup>-nitro- L-arginine, L-NA) inhibited ROS-induced leakage by 59% and PAF-induced leakage by 64%. The effects of L-NA and superoxide dismutase plus catalase on PAF-induced leakage were not additive. Systemic administration of mercaptoethylguanidine, a peroxynitrite scavenger, inhibited PAF-induced leakage by 60%. These results suggest that PAF-induced leakage may be mediated by an interaction between ROS and NO, perhaps through the formation of peroxynitrite or one of its products.