The experimental study was made on the influence of histamine and serotonin to the secretion of gastric juice and the development of peptic ulcer. Male rats weighing 150 to 200g. were used. The subjects were divided into 6 groups: the restraint group and the other five groups which were administered polymyxin B, reserpine, serotonin, prednisolone or ACTH, respectively. The volume and the acidity of the gastric juice decreased in polymixin B group, while the both remained unchanged in prednisolone group and increased in ACTH-group. The volume of the gastric juice decreased in restraint group and were unchanged in reserpine group, but the acidity increased in both group. Referring to the contents of histamine and serotonin in the glandular stomach after 4hrs treatment, no change occured other than that histamine reduced in polymyxin B group and serotonin increased in ACTH group. As for the relationship between the development of the ulcer and the contents of histamine and serotonin in the glandular stomach, the ulcer was observed in 70% of rats, which had been given the restraint 12hrs previously, without leaving any changes in histamine and serotonin contents. The evidence of the ulcer amounted to 80% in those which had been administered polymyxin B 4hrs previously. Histamine level turned out decreased in this group. The ulcer was found with the evidence of 70% in those which were sacrificed 4 days after the administration of prednisolone, accompanied with the decrease of both histamine and serotonin. The ulcer was found in 40%, 4 days after the administration of ACTH with the increase of serotonin, and 100%, 12hrs after the administration of reserpine with the decrease of both histamine and serotonin. The change of histamine was not observed in serotonin treated rats.