When type 1 diabetes mellitus was induced in rats by injecting streptozotocin, histidine decarboxylase expression was abnormally up-regulated in a transcriptional level, and 7 d after the injection, the enzyme activity was increased about 3-fold over the control (p < 0.05). When the diabetic rats were administered with insulin for 3 d, the increased histidine decarboxylase activity returned to a normal level in addition to normalization of the plasma glucose level. The plasma gastrin level in the fasting state was also significantly elevated in the diabetic rats, and the insulin treatment normalized the level. In the diabetic rats, fasting gastric acid output increased significantly and gastric pH was lowered. These results suggest that the gastric histidine decarboxylase activity and plasma gastrin level are increased in connection with the depletion of insulin in streptozotocin-induced diabetic rats, and gastric acid secretion is stimulated at a basal level, presumably due to increases in the concentrations of histamine and gastrin in oxyntic mucosa.