Coordinated studies have been carried out on the glucagon immunoreactive cells of the mammalian gastrointestinal tract (man, dog, rat), using electron microscopy, silver staining and immunocytochemistry. Parallel ultrastructural and immunocytochemical studies have been made with the semithin-thin serial section technique. The results indicate that while the glucagon cells of the oxyntic portion of the stomach are virtually indistinguishable from those of the pancreatic islets (A cells) those of the intestine (EG cells) are completely different. Proper identification of glucagon immunoreactive cells requires the application of morphological and silver staining techniques, at the ultrastructural level.