This study attempted to evaluate the relative roles of vessel distensibility and vessel recruitment in the changes in pulmonary blood volume induced by changes in pulmonary intravascular pressure. Pulmonary blood volume was determined by a double-injection indicator-dilution technique in eight anesthetized open-chest dogs with a right and left ventricular bypass which allowed independent control of pulmonary artery and left atrial pressures. In three dogs, measurements were also taken during reverse perfusion (from the left atrium to the pulmonary artery). During forward perfusion, when left atrial pressure was increased from 7 to 26 cm H 2 O, and, by reducing flow from 3.4 to 1.2 liters'/min, pulmonary artery pressure was maintained at 30 cm H 2 O so that recruitment could be considered constant, pulmonary blood volume did not change but remained at about 260 ml. Pulmonary blood volume also did not change during reverse perfusion when pulmonary artery pressure was increased from 8 to 27 cm H 2 O at a constant left atrial pressure (inflow pressure). By contrast, when pulmonary artery pressure was raised by increasing flow, so that vessel recruitment could occur, pulmonary blood volume increased 12 ml for each 1-cm H 2 O increase in pulmonary artery pressure at a constant left atrial pressure of 5 cm H 2 O, and pulmonary blood volume increased 7 ml for each 1-cm H 2 O increase in pulmonary artery pressure at a constant left atrial pressure of 33 cm H 2 O. Therefore, in the range of pressures that we explored, the role of large-vessel distensibility in the changes in pulmonary blood volume observed when intravascular pressures were raised was small.