Guazuma ulmifolia Lam., a member of the Sterculiaceae family, is used in folk medicine because of its antioxidant, antimicrobial and antihypertensive properties. Most of the research work carried out on this plant has focused on the bark because of its high concentration of antioxidant proanthocyanidins. The flowers and leaves of Guazuma ulmifolia, though less studied, are also used as a remedy for different conditions, such as kidney and gastrointestinal diseases, fever and diabetes. The aim of this study was to assess the gastroprotective effects of an aqueous suspension of the ethanolic extract from leaves and flowers of Guazuma ulmifolia in a model of acute gastric ulcer induced by diclofenac as ulcerogenic agent, using the proton pump inhibitor omeprazole as a protection reference. Therefore, the extract was administered two times orally to three groups of Wistar rats at doses of 500, 250 and 125mg/kg, with a 24-h interval between doses. Diclofenac (100mg/kg) was given 1h after the last administration of the extract. Pretreatment with Guazuma ulmifolia or omeprazole decreased the ulcerated area in a dose-dependent way. Myeloperoxidase activity as a marker of neutrophil infiltration was slightly reduced in vivo, whereas in vitro, anti-inflammatory activity was clearly inhibited in a dose-dependent way. The lowest doses of the extract significantly decreased the levels of lipoperoxides, and superoxide dismuthase activity increased to a similar extent as with omeprazole (P<0.001). Examination of glutathione metabolism reflected a significant rise in glutathione peroxidase activity at the highest dose of Guazuma ulmifolia. Finally, there was a faint elevation in prostaglandin E(2) levels with all doses, though the depletion induced by diclofenac could not be reverted. We conclude that the aerial parts of Guazuma ulmifolia protect gastric mucosa against the injurious effect of NSAIDs mainly by anti-inflammatory and radical-scavenging mechanisms.