Effects of propranolol on coronary artery were investigated in isolated dog coronary arteries and in isolated hearts of rats and guinea-pigs. Cumulative administration of dl-propranolol produced concentration-dependent relaxation of coronary arterial strips under potassium-contracture. There was no difference in relaxant potency between the optical isomers of propranolol. Lidocaine produced less relaxation than dl-propranolol, while atenolol and practolol scarcely affected the contraction. dl-Propranolol-induced relaxation was significantly restored by additional calcium or prevented by prior administration of calcium. In isolated hearts of rats and guinea-pigs which were perfused at a constant coronary inflow and paced at a constant rate, bolus administration of dl-propranolol produced dose-dependent falls in coronary perfusion pressure without significant changes in myocardial contractile force. Results indicate that propranolol may have relaxant activities on coronary arteries through inhibition of calcium influx in the cell membrane of coronary arterial smooth muscle.