The ingestion of dietary antioxidants, including vitamin C (VC), is suggested to play an important role in the prevention of gastric cancer associated with Helicobacter pylori (HP) infection. Recently, water extracts of Tochu (Du-zhong, Eucommia ulmoidea OLIVER) leaves (WETL) have been reported to have potent antioxidant and antimutagenic effects. The present study investigated the effect(s) of VC and WETL on gastric mucosal injury induced by ammonia and a VC deficient diet. Guinea pigs fed the water containing ammonia and/or a VC-deficient diet were simultaneously treated with WETL or VC. Intramucosal levels of thiobarubiturate reactive substances (TBARS), an index of lipid peroxidation, increased significantly in animals fed ammoniated water and VC-deficient diets. This was accompanied by accelerated cell proliferation and increases in immunohistochemical staining indices for oxidative stress-induced DNA adducts and strand breaks (e.g., BrdU-uptake, 8-OhdG, ssDNA and the TUNEL reaction). The administration of either WETL or VC to the ammoniated water and VC-deficient diets ameliorated the increases in intramucosal TBARS levels and labeling indices of BrdU, 8-OHdG, ssDNA and TUNEL, i.e., the levels were similar to those measured in the normal-fed control animals. These data suggest that insufficient VC ingestion may be an important risk factor for gastric cancer development in patients with HP infections. Furthermore, our results suggest that WETL or some constituent may contribute to the prevention of oxidative gastric injury that precedes carcinogenesis.