The light microscopic immunocytochemical localization of corticotropin-releasing factor (CRF) is described in the endocrine pancreas of several species representing the major classes of vertebrates: fishes (channel catfish, Ictalurus punctatus), amphibians (African clawed toad, Xenopus laevis), reptiles (chameleon, Anolis carolinensis), birds (chicken, Gallus domesticus), and several mammals (rat, mouse, cat, rhesus monkey, and man). The CRF-containing cells are scattered over the entire islet tissue in primates and cat, whereas in rat and mouse they are located at the periphery of the islets. In the chicken and catfish, the CRF-containing cells are found in a central location within islets and form larger clusters or cords. Single cells with CRF-like immunoreactivity are interspersed between acinar cells of the exocrine pancreas in all species studied. The CRF cells show a substantial topographical overlap with glucagon cells, but their precise identity and function remain to be determined.