Forty two carcinomas of the gallbladder and 25 mucinous cystadenocarcinomas of the pancreas were analyzed using silver stains and immunohistochemical techniques. Fourteen (33.3%) gallbladder carcinomas had argyrophil and argentaffin cells and 17 (40%) contained endocrine cells as shown by immunoperoxidase stains. The gallbladder tumors that had the largest number of endocrine cells were the well differentiated adenocarcinomas with colonic features. The most common endocrine cell in these tumors was the serotonin-containing (EC) cell followed by somatostatin-containing cells and cells that reacted to pancreatic polypeptide and gastrin. Intestinal metaplasia with pseudopyloric gland hyperplasia was present in the gallbladder mucosa adjacent to 11 carcinomas and had an endocrine cell population similar to that of the tumors. Endocrine cells were demonstrated in 18 (70%) of the 25 mucinous cystadenocarcinomas of the pancreas by the immunoperoxidase method although only 9 had argyrophil and argentaffin cells. The population of endocrine cells in these mucinous pancreatic tumors was similar to that found in gallbladder carcinomas. Endocrine cells were more numerous in areas with colonic-type glands, goblet cells and Paneth cells. The secretory products of the endocrine cells in these gallbladder and pancreatic tumors did not give rise to systemic endocrine manifestations. The presence of endocrine cells in these tumors can be explained on the basis of intestinal differentiation.