CGRP-immunoreactive nerve fibers were histologically stained in the endocardium and perivascular layer of coronary vessels of canine hearts. To examine the physiological role of the CGRP in the heart function, effects of exogeneous CGRP on the hearts were studied using canine isolated, blood-perfused heart preparations. CGRP exerted dose-related potent vasodilator effects with a minimal increase in the developed tension of the papillary muscle, but slightly decreased the sinoatrial rate. The vasodilator effects were unaffected by the pretreatment of either atropine or propranolol. These specific functional effects on the coronary artery are well in accordance with the anatomical localization of CGRP. Taken together, CGRP seems to play an important role in the regulation of coronary vascular tone, while it has only a small functional role in inotropism and chronotropism in canine hearts.