Eight healthy men were exposed to 5 days of continuous heavy exercise combined with caloric deficiency and sleep deprivation. Immediately after the stress period the fasting and postprandial intragastric bile acid concentration, pepsin concentration, and gastric juice acidity were measured. Compared with a control experiment performed 8 weeks later, the results from the stress period showed a sevenfold increase in the fasting concentration of intragastic bile acids (from 35 mumol/l to 256 mumol/l; p less than 0.02), there was a tendency towards an increase in the fasting intragastric pepsin concentration, and there was an increase (p less than 0.05) in the intragastric pH level throughout the 3rd postprandial hour. It appears that physical stress induces changes in the intragastric milieu that might dispose for mucosal lesions.