Adrenomedullin is a peptide of marked vasodilator activity first isolated from human pheochromocytoma and subsequently demonstrated in other mammalian tissues. Using a polyclonal antiserum against human adrenomedullin-(22-52) amide and the avidin-biotin peroxidase complex technique, we have demonstrated by light and electron microscopy that adrenomedullin-like immunoreactivity is widely distributed in the rat central nervous system. Western blotting of extracts of different brain regions demonstrated the fully processed peptide as the major form in the cerebellum, whereas a 14-kDa molecular species and a small amount of the 18-kDa propeptide were present in other brain regions. Immunoreactive neurons and processes were found in multipolar neurons and pyramidal cells of layers IV-VI of the cerebral cortex and their apical processes, as well as in a large number of telencephalic, diencephalic, mesencephalic, pontine and medullary nuclei. Cerebellar Purkinje cells and mossy terminal nerve fibers as well as neurons of the cerebellar nuclei were immunostained, as were neurons in area 9 of the anterior horn of the spinal cord. Immunoreactivity was also found in some vascular endothelial cells and surrounding processes that probably originated from perivascular glial cells. Electron microscopy confirmed the light microscopy findings and showed the reaction product in relation to neurofilaments and the external membrane of small mitochondria. Immunoreactive terminal boutons were occasionally seen. The distribution of adrenomedullin-like immunoreactivity in the central nervous system suggests that it has a significant role in neuronal function as well as in the regulation of regional blood flow.