The gastroprotective effect of the triterpene oleanolic acid (OA) was assessed on gastric ulceration in rats. The effect of a single oral dose of OA was evaluated at 50, 100 and 200 mg kg(-1) in the following models: pylorus ligature (Shay), and aspirin- and ethanol-induced gastric ulcers. A single oral administration of OA at doses of 50, 100 and 200 mg kg-' inhibited the appearance of gastric lesions induced by ethanol, aspirin and pylorus ligature. In the pylorus ligature and aspirin models, the effect of OA at the selected concentrations was comparable with that of ranitidine at 50 mg kg(-1). In the ethanol-induced gastric lesion model, OA showed a dose-dependent activity, and at 100 and 200 mg kg(-1) was as active as omeprazole at 20 mg kg(-1). The effect of OA, its acetylated and methoxylated derivatives, oleanonic acid and its methyl ester were assessed on HCI/ethanol-induced ulcers in mice at 200 mg kg(-1). OA and its methoxylated (OAM) and acetylated (OAAM, OAA) derivatives proved to be active in this animal model. The semisynthetic derivatives OAM and OAAM had the greatest gastroprotective activity, but their effect was not significantly greater than OA. In an acute toxicity test on mice, intraperitoneal administration of OA showed no toxicity at doses up to 600 mg kg(-1).