Nitric oxide (NO) is a key molecule involved in many physiological processes. To characterize its roles in the tolerance of Arabidopsis thaliana to ultraviolet-B (UV-B), we investigated the effect of a reduced endogenous NO level on oxidative damage to wild-type and mutant (Atnoa1) plants. Under irradiation, hydrogen peroxide was accumulated more in mutant leaves than in the wild type. However, the amounts of UV-B-absorbing compounds (flavonoids and anthocyanin) and the activities of two antioxidant enzymes-catalase (CAT, EC 1.11.1.6) and ascorbate peroxidase (APX, EC 1.11.1.11)-were lower in leaves of the former. Supplementing with sodium nitro-prusside, an NO donor, could alleviate the oxidative damage to mutant leaves by increasing flavonoid and anthocyanin contents and enzyme activities. In comparison, $N^{omega}-nitro-{iota}-arginine$, an inhibitor of nitric oxide synthase, had the opposite effects on oxidation resistance in wild-type leaves. All these results suggest that nitric oxide acts as a signal for an active oxygen-scavenging system that protects plants from oxidative stress induced by UV-B irradiation.