A new method has been described for measuring the pressure and resistance to blood flow in the pulmonary arteries, capillaries, and veins. Studies were performed in dog isolated lung lobes perfused at constant flow with blood from a donor dog. Pulmonary artery and vein volume and total lobar blood volume were measured by the ether plethysmograph and dyedilution techniques. The longitudinal distribution of vascular resistance was determined by analyzing the decrease in perfusion pressure caused by a bolus of low viscosity liquid introduced into the vascular inflow of the lobe. The pulmonary arteries were responsible for 46% of total lobar vascular resistance, whereas the pulmonary capillaries and veins accounted for 34 and 20% of total lobar vascular resistance respectively. Vascular resistance was 322 dynes .sec.cm(-5)/ml of vessel in the lobar pulmonary arteries, 112 dynes.sec.cm(-5)/ml in the pulmonary capillaries, and 115 dynes.sec.cm(-5)/ml in the lobar pulmonary veins. Peak vascular resistivity (resistance per milliliter of volume) was in an area 2 ml proximal to the capillary bed, but resistivity was high throughout the pulmonary arterial tree. The pulmonary arteries accounted for approximately 50% of vascular resistance upstream from the sluice point when alveolar pressure exceeded venous pressure. The method described provides the first measurements of pulmonary capillary pressure. Mid-capillary pressure averaged 13.3 cm H(2)O, pulmonary artery pressure averaged 20.4 cm H(2)O, and pulmonary vein pressure averaged 9.2 cm H(2)O. These techniques also provide a way of analyzing arterial, capillary, and venous responses to various pharmacologic and physiologic stimuli.