1. Distension of the pulmonary vein-left atrial junctions caused an increase in heart rate and a transient decrease in mean arterial pressure. 2. Section of the brain stem at the level of the inferior cerebellar peduncle (rostral medulla) caused a decrease in mean arterial pressure. 3. Section of the rostral medulla had no effect on either the magnitude or the time course of the reflex response to pulmonary vein distension. 4. Administration of propranolol after section in the rostral medulla reduced the reflex increase in heart rate in response to pulmonary vein distension by an amount similar to that previously described in intact and decerebrate animals. 5. Bilateral cervial vagotomy prevented the reflex response to pulmonary vein distension.