The immunohistochemical localization of calcitonin gene-related peptide was examined, at both light and electron microscopic levels, in the pancreas of various vertebrates, including the eel, bullfrog, turtle, chicken, mouse, rat, guinea pig, dog, monkey, and human. Immunoreactive staining was observed in nerve fibers in every animal species examined, but positive endocrine cells were limited to the rat, monkey, and human. The density of the positive endocrine cells varied considerably among the three species (monkey > rat > human). Positive nerve fibers were distributed throughout the parenchyma, being particularly rich around pancreatic ducts, and near large or small blood vessels. In four species (eel, mouse, rat, and dog), positive nerve fibers formed a dense network in the islet region. There were positive varicose nerve fibers around exocrine cells. These fibers, varying in density in different species (relatively high in the eel, bullfrog, and rat), were sometimes adjacent to acinar cells. At the electron microscopic level, positive nerve terminals were often demonstrated in close apposition to the outer membrane of acinar cells. The eel pancreas revealed an exceptional pattern of staining in neuronal cell bodies that were scattered in the interlobular connective tissue. Despite these anatomical differences, the omnipresence of this peptide suggests its essential role(s) in the pancreas.