The histological localization was investigated of adrenomedullin (AM), a novel vasorelaxant peptide originally isolated from human pheochromocytoma. The immunohistological distribution was examined of AM in human, rat, and porcine tissues using a polyclonal antibody to a fragment comprising C-terminal amino acids 40-52 of human adrenomedullin [AM(40-52)NH2]. Almost all of the human pheochromocytoma and normal adrenal medullary cells of all three species were immunostained and found to be intensely positive for AM. Furthermore, AM-immunoreactive cells were present in the pancreatic islets, gastrointestinal neuroendocrine system, anterior pituitary, and choroid plexus with some degree of interspecies heterogeneity. These findings indicate that AM-immunoreactive cells are widely distributed in the endocrine and neuroendocrine system, suggesting that AM plays some important role in the control of systemic and local circulation and also of humoral secretion.