From venous tracer-dilution curves, after 57 pulse injections of "NaCl and [13II]albumin (RISA) into the arterial inflow of nine isolated canine lung lobes perfused with blood (hematocrit = 0.24-0.44) under zone III conditions, we calculated area-weighted fractional extractions (E3) and capillary permeability (P)-surface area (S) products (PS) for24Na at plasma flows (Fp) ranging from 4.1 to 40.1 ml»min"1«g"1 dry weight. In six of the lobes, 35 separate injections of RISA and [l25I]iodoanti- pyrine permitted calculation of pulmonary blood volume (PBV) and extravascular lung water (EVW). Our experimental preparation allowed us to evaluate, independently, the effects of flow and perfusion pressure on the measurements of PS, PBV, and EVW. PS increased as Fp was raised, but at any given Fp, PS remained constant despite large changes in pulmonary arterial (PA, range 5-38 mm Hg) and venous (Pv, range 2-30 mm Hg) pressures. Mean EVW (2.8 ± 1.5 ml/g, n = 35) was unaffected by changes in Fp, PA, or Pv. An increase of 52% in PBV occurred as mean PA increased from 11 to 33 nun Hg. This increase in PBV was due to disterition of small intrapulmonary vessels. Since EVW remained unchanged, there was no evidence for vascular recruitment in these lobes (that is, no change in S), and assuming no change in permeability, we concluded that the flow-dependent changes in PS are related to underestimates of E3 and PS at low Fp. At high Fp, mean PS was 3.6 ml»min∼1«g∼I dry weight. If S for the lung is 3000 cm2/g, then pulmonary capillary permeability for sodium is about 3.8 × 10∼6 cm/sec, which is considerably lower than the 3.1 × 10"5 cm/sec that we have reported for myocardial capillaries. Circ Res 46: 669-680, 1980