The immunoreactive gastrin (IRG) and somatostatin (IRS) contents in gastric mucosa were measured from the same biopsy specimen of the same patients with duodenal ulcer (DU) at the active stage and healing stage, and compared to those of patients with fundic gland polyposis (FP) and endoscopically normal subjects whose gastric mucosa had only slight atrophic change (Control). The IRS in both the antrum and the gastric body of DU were significantly lower than those of the other two groups, and those showed no difference between the two stages. In all groups, there was a significant positive relation between the IRG and IRS in the antrum. In DU, particularly at the active stage, the relative decrease of the IRS against the IRG was prominent compared to the other two groups. In FP, which has similar background gastric mucosa and ability of acid output to those of DU, it was found that somatostatin was secreted sufficient to control gastrin secretion and acid output. Whereas in DU, secretion of somatostatin was reduced and, particularly at the active stage, it was considered that somatostatin, which could control increased gastrin secretion and increased acid output, was not secreted.