Concentrations of free and total hydrogen ions, total protein and pepsin were measured in gastric juice fractions collected during basal secretion and upon stimulation by graded doses of pentagastrin administered intravenously. Undissociated hydrogen ion and non-pepsin protein concentrations were calculated as derived quantities.The studies were carried out in nine patients with duodenal ulcer both before and after truncal vagotomy. It was found that after vagotomy the undissociated hydrogen ion concentration was significantly lower and non-pepsin protein higher than before the operation. No correlation was found between the two quantities both before and after vagotomy. It was concluded that in duodenal ulcer patients either not all non-pepsin protein takes part in buffering of hydrogen ions secreted by parietal cells, or that non-protein buffers play a more important role.