Abstract
1. Stimulation of left atrial receptors by distension of the junctions between the pulmonary veins and the left atrium is known to cause a reflex increase in heart rate. It was suggested that the efferent path of this reflex was solely in the sympathetic nerves to the heart but more recently the existence of a vagal efferent component has been postulated by Albrook, Bennion & Ledsome (1972). 3. The junctions between the pulmonary veins and the levt atrium were distended before and after the administration of I.C.I. 66082 and bretylium tosylate. The response of an increase in heart rate was significantly decreased after the administration of I.C.I. 66082 (5 mg/kg) and abolished after the administration of bretylium tosylate (10 mg/kg). 3. It is concluded that the efferent pathway of the reflex is solely in the sympathetic nerves to the heart.