Gastroscopy with gastric biopsy was performed in 109 individuals aged 25–71 years. Activities of three antioxidant enzymes were assayed in biopsy specimens: Cu/Zn superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSH-Px). Patients were classified according to the endoscopic and histological findings in the following groups: normal findings (N), superficial gastritis (SG), mild (MAG) and severe (SAG) atrophic gastritis, gastritis after partial gastrectomy (PGG), hyperplastic polyp (HP), and gastric adenoma (A). Compared with the N group, increased activity of SOD was found in groups SG (+37%), PGG (+67%) and A (+35%), increased CAT activity in PGG (+40%), and increased GSH-Px activity in groups SG (+57%), SAG (+46%), PGG (+185%), HP (+50%) and A (+50%). Increased activity of antioxidant enzymes could be induced by higher concentrations of superoxide anion radicals, hydrogen peroxide and lipid peroxides, produced by phagocytic leucocytes or by polyunsaturated fatty acid in cellular membranes of gastric mucosa. The relation of reactive oxygen species to the induction of precancerous conditions and to carcinogenesis of the stomach requires further study.