A horizontal isolated lung preparation has been used to examine the transmission of pulsatile blood pressure and flow. Arterial and venous pressure and flows were measured with pressure transducers and electromagnetic flowmeters. After a sudden increase in pulmonary arterial pressure, delays of several seconds were observed before venous flow began to increase. Flow then increased slowly over a period of 1 to 9 seconds to a new steady level. The transmission of pulsatile flow was independent of the outlet pressure and identical for perfusion in the forward and reverse directions. Flow transmission fell from about 75% at a frequency of 0.03 cps to approximately 10% at a frequency of 1 cps. Pressure transmission down to the collapsible vessels was measured when alveolar pressure exceeded venous pressure. At 0.1 cps approximately 50% of the incident pressure wave was transmitted, and at 1 cps this was reduced to 30%. It was concluded that the lung passed well only the low-frequency components of the applied input and that in a vertical lung only about 30% of the mean to peak pressure pulse would be transmitted down to the small vessels and therefore affect the distribution of blood flow.