In 15 undialyzed (UD) patients and 26 hemodialyzed (HD) patients, (1) basal and test meal-stimulated gastrin concentrations, (2) basal and pentagastrin-stimulated gastric acid outputs, and (3) endoscopic examinations were studied. Also studied were the morphological and functional differences of the gastrointestinal tract between UD and HD patients. HD patients had lower gastric acid outputs and higher circulating gastrin levels in the fasting state. After a test meal, the peak increment of serum gastrin in UD and HD patients occurred 30 and 60 min later, respectively, and the response was prolonged in each group. Endoscopic findings showed that the incidence of abnormalities in each group was very similar, that is, the most predominant lesions in each group were hemorrhagic and atrophic gastritis. The data suggested that the response to gastrin of parietal cell and the defensive mechanism of gastrointestinal mucosa in HD patients may be impaired.