Experiments were designed to evaluate endothelium-dependent responses of pulmonary arteries following prolonged exposure to oxygen-derived free radicals. Rings of canine pulmonary arteries with and without endothelium were suspended for measurement of isometric force in organ chambers and incubated with xanthine (10-4 M) plus xanthine oxidase (0.015 U/ml) for 1 h in the absence and presence of either superoxide dismutase (SOD, 150 U/ml), catalase (1,200 U/ml), deferoxamine (10-3 M), or a combination of all three scavengers. Xanthine plus xanthine oxidase caused significantly greater contractions of rings without compared with those with endothelium. In rings with endothelium, contractions were reduced by SOD or catalase but not by deferoxamine. Following 1 h of exposure to xanthine plus xanthine oxidase, endothelium-dependent relaxations to ADP were reduced but not those to bradykinin or the calcium ionophore A-23187 (calcimycin). Relaxations to ADP were not corrected by incubation with the antioxidants used singly or in combination during the exposure to xanthine plus xanthine oxidase. These results suggest that oxygen-derived free radicals generated from exogenously applied xanthine plus xanthine oxidase cause contractions of canine pulmonary arteries. In addition, even when contractions of rings with endothelium were prevented by SOD and catalase, subsequent expression of some but not all endothelium-dependent relaxations were reduced. Therefore, scavenging of oxygen-derived free radicals may prevent some but not all of the vascular injury caused by oxygen-derived free radicals. Copyright © 1996 the American Physiological Society.