The canine lung lobe was embolized with 100-micron glass beads before lobectomy and blood anticoagulation. The lobe was isolated, ventilated, and pump-perfused with blood at an arterial pressure (Pa) of about 50 (high pressure, HP, n = 9) or 25 Torr (low pressure, LP, n = 9). Rus/PVR, the ratio of upstream (Rus) to total lobar vascular resistance (PVR), was determined by venous occlusion and the isogravimetric capillary pressure technique. The capillary filtration coefficient (Kf), an index of vascular permeability, was obtained from rate of lobe weight gain during stepwise capillary pressure (Pc) elevation. The embolized lobes became more edematous than nonembolized controls, (C, n = 11), (P less than 0.05), with Kf values of 0.20 +/- 0.04, 0.25 +/- 0.06, and 0.07 +/- 0.01 ml X min-1 X Torr-1 X 100 X g-1 in LP, HP, and C, respectively (P less than 0.05). The greater Rus/PVR in embolized lobes (P less than 0.05) protected the microvessels and, although Pc was greater in HP than in controls (P less than 0.05), Pc did not differ between HP and LP (P greater than 0.05). Although indexes of permeability did not differ between embolized groups (P greater than 0.05), HP became more edematous than LP (P less than 0.05). The greater edema in HP did not appear due to a greater imbalance of Starling forces across the microvessel wall or to vascular recruitment. At constant Pc and venous pressure, elevating Pa from 25 to 50 Torr in embolized lobes resulted in greater edema to suggest fluid filtration from precapillary vessels.