Experiments were designed to determine the role of oxygen-derived free radicals in modulating contractions of vascular smooth muscle and endothelium-mediated relaxations to acetylcholine. The effects of generating or scavenging these radicals were studied in rings of canine coronary arteries suspended for isometric tension recording. Xanthine oxidase plus xanthine caused relaxations, which were greater in rings with endothelium than in rings without endothelium; the relaxations were not affected by superoxide dismutase or mannitol, but could be prevented by catalase. Xanthine oxidase plus xanthine depressed endothelium-mediated relaxations to acetylcholine; this effect was prevented by superoxide dismutase, but was not affected by catalase or mannitol. Exogenous hydrogen peroxide induced catalase-sensitive relaxations, which were depressed by the removal of the endothelium. Superoxide dismutase evoked catalase-sensitive relaxations only in rings with endothelium. Endothelium-mediated relaxations to acetylcholine were slightly depressed by superoxide dismutase or catalase alone; the combination of the two enzymes or mannitol caused a major shift to the right of the concentration-response curve to acetylcholine. In rings without endothelium, relaxations caused by sodium nitroprusside were not affected by the scavengers (alone or in combination) but were augmented by xanthine oxidase plus xanthine. These data suggest that the endothelium-derived relaxing factor released by acetylcholine is not likely to be an oxygen-derived free radical; hydrogen peroxide has a direct inhibitory action on coronary arterial smooth muscle and triggers endothelium-dependent relaxations; and superoxide anions depress and hydroxyl radicals facilitate endothelium-dependent relaxations caused by activation of muscarinic receptors.