Direct measurements were made of the collateral flow to the left lower lobe of the lung after occlusion of the left pulmonary artery in anesthetized, open-chest dogs. Immediately after ligation of the pulmonary artery flow ranged from 4.4 to 9 cc. per minute and varied with the height of the systemic blood pressure. The flow at a given blood pressure did not increase during 2 to 4 hours of observation. After chronic ligation of the left pulmonary artery, the flow increased, ranging from 68 to 376 cc. per minute at the end of approximately one year.