The anti-ulcerogenic and anti-oxidant effects of various flavonoids have been frequently reported. We investigated the cytoprotective properties of quercetin, a natural flavone, in gastric mucosal injury induced by 50% ethanol, since in this experimental model the pathogenesis of the lesions has been related with production of reactive oxygen species. The involvement of neutrophil infiltration and the capacity of this flavonoid to restrain the oxidative process produced in the gastric tissue after ethanol administration were also investigated. Oral pretreatment with the highest dose of quercetin (200 mg/kg), 120 min before absolute ethanol was the most effective anti-ulcer treatment. Thiobarbituric acid reactive substances in the gastric mucosa, an index of lipid peroxidation, were increased by ethanol injury, but the increase was inhibited by the administration of 200 mg/kg of quercetin. This dose also induced a significant enhancement in the levels of mucosal non-protein SH compounds (important anti-oxidant agents) and in glutathione peroxidase activity. Exposure of the gastric mucosa to 50% ethanol induced a significant increase in myeloperoxidase activity, an index of neutrophil infiltration. However, quercetin was not able to modify the increase in enzymatic activity generated by the necrotizing agent. The activity of superoxide dismutase enzyme involved in several antioxidant processes was also not significantly modified after quercetin treatment. These results suggest that the anti-ulcer activity of quercetin in this experimental model could be partly explained by the inhibition of lipid peroxidation, through decrease of reactive oxygen metabolites. However, the inhibition of neutrophil infiltration or the increase of superoxide dismutase activity does not appear to be involved in gastroprotective effect of this flavonoid.