We investigated by immunocytochemistry (ICC) the distribution in the rat heart of adrenomedullin (AM), a potent and long-lasting hypotensive peptide which is expressed in the cardiovascular system, where it is known to play a major regulatory role. Hearts were collected from adult male Sprague-Dawley rats, and were perfused for 20 min, according to the Langendorff technique, with endothelin-1 (ET-1) or the mast cell-degranulator compound 48/80. Hearts were frozen, and ICC was performed using standard techniques and a specific anti-rat AM1-50 antibody. We confirmed the presence of a low AM-immunoreactivity in cardiomyocytes and cardiac fibroblasts, as well as in endothelial and smooth muscle cells of coronary vessels. Moreover, we provided evidence of the presence in both atria and ventricles of sparse voluminous AM-positive cells, mainly located near coronary vessels. These cells had the same juxtavascular location of toluidine blue-positive mast cells and their number decreased upon acute exposure to the 48/80 compound. However, ICC showed that in these cells AM was always colocalized with atrial and brain natriuretic peptides. Moreover, AM-storing cells were also positive to MyHC-Apla2, indicating that they share some phenotypic features with immature smooth muscle cells. The number of AM-storing cells underwent a dramatic decrease in response to the potent vasoconstrictor ET-1, suggesting an acute release of stored vasodilatory AM aimed at counteracting coronary constriction. Taken together, our present findings support the hypothesis that these cells may represent a novel subset of endocrine cells, strategically located near blood vessels in the mammalian heart, where they can release vasoactive peptides.