The morphology and histochemistry of gastroduodenal endocrine tumors from 16 patients were studied. All patients underwent operation, in most cases with a preoperative diagnosis of nonendocrine tumor, ulcer, o polyp(s). The argentaffin reaction was positive in three tumors, and the Hellerström--Hellman argyrophil reaction was positive in four tumors. All tumors reacted positively to the Grimelius argyrophil stain, and 13 were positive with the Sevier--Munger argyrophil stain. Gastrin immunoreactivity was found in eight tumors, and substance-P immunoreactivity in seven tumors. No enteroglucagon, adrenal cortex hormone, or pancreatic polypeptide was observed in any of the tumors. Three patients with Sevier--Munger-positive gastric tumors had concurrent pernicious anemia, and 2 patients with gastrin-immunoreactive tumors had acute or chronic gastroduodenal ulceration. The results indicate that the gastroduodenal endocrine tumor as a rule gives no endocrine symptoms and that the tumor type is an unexpected finding at operation. The tumors may contain gastrin, substance P, somatostatin, and serotonin and tend to be multihormonal.