Rats bred in an animal house with a light and obscurity artificial cycle of 12/12 h at 25° C were restrained for 24 h in metal tubes at 4 different temperatures 19°, 22°, 25° and 28° C. The frequency of gastric lesions in each group was respectively of 100%; 67%; 17% and 25%. Thus, small fluctuations of the ambient temperature around the average have a dramatic influence on the ulcer ratio. The cold exposure increases the susceptibility. The temperature fluctuation must be an important factor in the seasonal variations of ulcer frequency in various experimental conditions. A strict control of the temperature must be obtained to get a reproducible incidence of restraint ulcer.