The numbers of immunoreactive gastrin and somatostatin cells in gastric and duodenal mucosal biopsy specimens from dyspeptic patients with duodenal ulcers and dyspeptic controls without ulcers were calculated using a morphometric method. The levels of gastrin and somatiotatin in the tissue were also measured by the radioimmunssay. The results showed no significant difference in the number of G cells and the level of gaslrln in the tisse between the ulcer and nun-ulcer groups. However, the number of D ceils and the level of somaosutin in the tissue in ulcer patlems were remarkably reduced in comparison with those in non-ulcer patients (P<0.01 and P<0.05, respectively). The G: D cells and gastrin : somatostatin rsatios in ulcer patients were much higher than those in the non-ulcer control group It is considered that the reduction of D cells and the relative lack of somatostafin in duodenal ulcer patients might have a role in the mechanism of the duodenal ulceration.