Bumepidil (also known as CS-611) is intravenously administered coronary vasodilator with antiarrhythmic action. In anesthetized, open-chest dogs Bumepidil in doses of 0.1--1 mg/kg i.v. decreased systemic blood pressure, coronary resistance and arterio-venous oxygen difference and increased coronary sinus outflow in a dose-related manner, but myocardial oxygen consumption was virtually unchanged. At 0.3 mg/kg i.v. of the drug, coronary sinus outflow was doubled, but heart rate and AV conduction time were not changed. Bumepidil shortened the action potential duration (APD) of each tissue, especially of Purkinje fibers, without any significant alterations in the resting membrane potential, action potential amplitude and maximum rate of rising. Bumepidil can be characterized as a coronary dilator virtually devoid of cardiac actions in doses sufficiently increasing coronary blood flow.