Twelve healthy subjects were exposed to a 4-day period of hard physical exercise, calorie supply deficiency, and severe sleep deprivation. The basal acid output (BAO), the sham-feeding-induced acid output (MAOsh), and the pentagastrin-stimulated acid output (MAOpg) were measured immediately after this stress period and in a control experiment performed several weeks later. The stress induced a threefold increase in the median BAO and an increase (p less than 0.05) in the MAOsh, which, however, was not significantly elevated when basal-subtracted. MAOpg was unchanged. In contrast to acid, pepsin output was not influenced by stress. The human pancreatic polypeptide (hPP) level in serum increased twofold after the stress. The integrated hPP response induced by modified sham feeding was higher (p = 0.02) after the stress than in the control experiment. The results show that physical stress has separate influence on the gastric secretion of acid and pepsin.