The purpose of this study was to assess the preventive effect of pre-mild whole-body heating (WBH) on gastric ulcer induced by restraint and water-immersion stress. The ulcer index and ulcer area ratio in rats exposed to restraint and water-immersion stress were significantly decreased (p < 0.05 for both) after pre-treatment with mild WBH, compared with non-pre-treated rats. Mortality of rats among the pre-treated with mild WBH (0%) was lower than in the control group (33%). The concentration of HSP 70f in the stomach (both fundic and pyloric mucosal areas) of rats pre-treated with mild WBH was significantly higher than in animals exposed to restraint and water-immersion stress alone (p < 0.05) before exposed to stress, but was not significantly higher immediately after stress or 1 or 3 days later. The HSP 70f content of peripheral lymphocytes was increased by the pre-treatment with mild WBH. These results suggest that HSP 70f induced by pre-treatment with mild hyperthermia protects against more severe stress due to restraint and water-immersion, thereby preventing gastric ulcer formation. Pre-treatment with mild WBH is the safest cytoprotective method through the accumulation of HSP 70f. The concentration of HSP 70f in peripheral lymphocytes may be a useful clinical laboratory indicator for assessing the level of HSP 70f as having cytoprotective activity.