We previously suggested that hydroxyl free radical (•OH) production may play a role in carcinogenesis induced by N-methyl-NЈ-nitro-N-nitrosoguanidine (MNNG). MNNGinduced gastric cancer in rats and human gastric carcinoma occur most often in the antral mucosa and rarely in the normal fundic mucosa. We hypothesized that regional differences in anti-oxidant activity may be responsible. In the present study, we examined anti-oxidant activity by comparing the relative rates of reduction of a nitroxide free radical, 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy (Tempol), in the antral and fundic mucosa of male Wistar rats using ESR. The relative rate of Tempol reduction was significantly slower in the antral portion of the wall than in the fundic portion when Tempol (4 ؋ 10 ؊6 mole/mg wet weight of gastric wall) in HEPES buffer (pH 7.4) was spread over the mucosal surface of a section of the gastric wall. Addition of a sulfhydryl group modulator, N-ethylmaleimide, to the mucosal surface before treatment with Tempol removed the significant difference observed in the rates of reduction in the antral and fundic portions of the gastric wall. No signals were detected in the muscle layer. Our results indicate that the relative rate of free radical reduction by sulfhydryl groups was significantly slower in the antral mucosa than in the fundic mucosa. We therefore conclude that a regional difference in the rates of reduction of free radicals by sulfhydryl groups may result in the site susceptible to development of MNNGinduced gastric cancer.