A method is described for the measurement of the collateral pulmonary blood flow in the thoracotomized dog by utilization of a flow-meter which measures flow into an aortic sac from which the bronchial arteries arise. This preparation maintains the normal perfusing pressures and diminishes phase changes in the arterial pulse waves. The bronchial vascular resistance is increased by an increase in the insufflation air pressure, by bilateral vagotomy, or by the administration of l-epinephrine into the bronchial arterial circulation. The bronchial vascular resistance was decreased after a transitory interruption of the bronchial arterial blood flow (reactive hyperemia). After administration of serotonin in the bronchial artery, a transient increase in resistance was followed by a marked decrease. Intrapulmonary vascular injections produced only a decrease in resistance. The present data demonstrate the sensitivity of the preparation described.