The role of the carotid sinus reflex in control of the blood flow in the left circumflex coronary artery was studied in nine open-chest dogs anesthetized with chloralose. Flow was measured with an electromagnetic flowmeter. The effects of bilateral common carotid occlusion were examined in a three-part experiment in each dog. (1) Under vagotomy (control) conditions, occlusion of both common carotid arteries resulted in tachycardia, increased blood pressure, and a decrease in coronary artery resistance. (2) After beta-receptor blockade with propranolol, 1.0 mg/kg, carotid sinus hypotension still resulted in increased blood pressure but the tachycardia was blocked, and coronary artery resistance increased. (3) Following rapid cardiac sympathectomy, carotid sinus hypotension still resulted in a reflex rise in systemic blood pressure but coronary resistance remained steady. This demonstrated the reflex nature of the increased coronary resistance observed after beta-receptor blockade. It is concluded that the coronary circulation is under reflex control and that sympathetic coronary artery vasomotion is a part of a carotid sinus reflex.