Simultaneous measurements of instantaneous pulmonary arterial input pressure and flow were compared with lung capillary blood flow in 13 closed-chested dogs. Vasoconstrictor effects of hypoxia and serotonin infusion were shown to cause major alterations in the normal input impedance spectrum of the pulmonary arterial system. In spite of this, the transmission ratio of flow pulsatility from the main pulmonary artery to the lung capillaries remained unaltered, indicating that the time constant of the pulmonary arterial system is self-regulating.