Hyperbaric oxygenation has been used as the method of treatment in several ischemic diseases, but its effectiveness still remains controversial. The authors investigated the effect of hyperbaric oxygenation on ischemia-reperfusion injury of the small intestine using a rat model. Wistar King A Makino (WKAM) rats were subjected to 120 minutes of superior mesenteric artery occlusion before reperfusion, with 90 minutes of hyperbaric oxygenation (two absolute atmospheric pressure in an experimental hyperbaric chamber) during ischemia in group A and immediately after reperfusion in group B, and no hyperbaric oxygen was provided to group C. Jejunal samples 1.5 cm in length were taken at the end of ischemia in all groups, at 30 minutes after reperfusion in groups A and C, and at 120 minutes after reperfusion in groups B and C, for the measurement of adenine nucleotides (high-performance liquid chromatography method) and for histological examination (hematoxylineosin [HE] staining). The survival rate was significantly higher in group A than in group C. The amount of adenosine triphosphate in the samples was not significantly different among the three groups, whereas the energy charge at the end of ischemia was significantly higher in group A than in group C. Histologically, the damage to the mucosa and the longitudinal muscle layer decreased in group A compared with that observed in groups B and C. These results suggest that hyperbaric oxygenation during ischemia is able to ameliorate ischemia-reperfusion injury in the rat small intestine.