In 20 patients with duodenal ulcer or gastric ulcer the secretion of pepsin following a continuous intravenous infusion of pentagastrin in a dose of 0.1 μg/kg/min and of histamine in a dose of 0.7 μg/kg/min respectively, has been investigated. In all cases stimulation resulted in a considerable and constant increase in the secretion of pepsin above the basal values in output as well as in concentration. The total output of pepsin was found to be slightly higher follosing administration of histamine, than following administration of pentagastrin, but this difference is not statistically significant. The maximal values of output and concentration are obtained slightly sooner following pentagastrin stimulation than following histamine stimulation, but no other differences have been found. It is concluded that stimulation with pentagastrin and histamine, respectively, in the doses mentioned above produce in man pepsin secretions of similar characte