1. Maximal acid output after pentagastrin stimulation, and fasting and postprandial serum gastrin concentrations were determined in 25 normal subjects, 30 patients with corpus gastric ulcers, 10 patients with prepyloric ulcers and 30 patients with both duodenal and gastric ulcers. 2. Corpus ulcers and prepyloric ulcers formed one distinct group. Maximal acid output was abnormally low in the corpus ulcer patients and no different from normal in prepyloric ulcer patients, whereas fasting serum gastrin and postprandial integrated gastrin response was abnormally high in the former and no different from the normal in the latter. Furthermore, as in the normal subjects, a significant negative correlation between maximal acid output expressed in mmol h(-1) kg(-1) body weight and postprandial integrated gastrin response was observed in the corpus and prepyloric ulcer patients taken as a group. 3. In complete contrast patients with both duodenal and gastric ulcers, in whom postprandial integrated gastrin response was statistically highest amongst the three types of gastric ulcers, had a significantly positive correlation between maximal acid output and the integrated gastrin response. 4. These findings suggest the operation of different pathophysiological mechanisms in gastric ulcers with and without associated duodenal ulcers.