We measured the pH of the residual pool of gastric juice during routine endoscopy of the stomach and duodenum in 27 patients. After injecting 3 µg/kg pentagastrin intravenously additional samples were obtained 10-12 minutes later for determination of pH. No complications occurred in spite of the rapid administration of this dose. Analysis of the “basal” determinations revealed a large overlap between groups but the pH of the “poststimulated” specimens proved much more accurate in the division of patients into “low,” “normal,” or “high” secretor groups. When the pH of the stimulated specimens was converted to hydrogen ion concentration and then calculated to yield an estimate of peak acid output, the results were almost identical to the standard measurements of gastric secretory output after subcutaneous pentagastrin.