Pulmonary vascular pressure, blood flow, and blood volume were measured in the supine and sitting positions in eight subjects with localized lung carcinoma associated with moderate airway obstruction. Supine cardiac output, pulmonary wedge (Ppw) and artery (Ppa) pressure, and pulmonary vascular resistance (PVR) were normal. Circulatory changes in sitting position were also normal: heart rate increased 13 +/- 9% (mean +/- SD); stroke volume fell 21 +/- 15%; cardiac output fell 13 +/- 19%; and arteriovenous O2 difference increased 37 +/- 21%. Neither the difference between mean Ppa and mean Ppw nor the rise of PVR from 92 +/- 25 to 122 +/- 49 dyn.s.cm-5 in sitting position were significant. Pulmonary blood volume (PBV) as measured by a dye-bolus-injection technique fell from 517 +/- 122 ml supine to 360 +/- 43 ml sitting (P less than 0.01). This decrease is best explained by closure of alveolar vessels in the upper part of the lung and by the concomitant cessation of flow in corresponding extra-alveolar vessels, which would prevent distribution of dye in the region. Circumstantial evidence suggests the latter vessels remain open under the large expanding stresses that prevail in the upper lung.