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We studied the effects of an increase in pulmonary blood flow (PBF) on steady-state lung lymph flow (QL) and protein transport in anesthetized dogs (n = 7) to estimate the effect of vascular recruitment in zone 3 on transvascular filtration. At the end of each experiment, we increased left atrial pressure to 25-30 mmHg using a balloon catheter and obtained a washdown of the lymph protein concentration. PBF was increased with an extracorporeal circuit, which pumped blood from the left to the right atrium, and increases in pulmonary capillary pressure (Pc) were minimized by lowering left atrial pressure. PBF was measured by thermodilution, and Pc was measured by transient analysis of arterial occlusion pressure with a Swan-Ganz catheter. PBF increases averaged 78% with increases ranging from 36 to 118%. Pc increases ranged from 0.5 to 6.3 mmHg, and QL increases averaged 31% with changes ranging from -2 to +138%. We observed a 16% increase in QL for each 1-mmHg increase in Pc during increased PBF, which was comparable to the relationship previously observed after an increased left atrial pressure. Lymph-to-plasma total protein concentration ratios (CL/CP) decreased from 0.71 +/- 0.04 to 0.625 +/- 0.06 during increased PBF. The relationship between CL/CP, QL, and Pc for both increased blood flow and increased left atrial pressure were within the expected range for increased pressure alone. These data suggest that there was minimal vascular recruitment for transvascular filtration in zone 3 when pulmonary blood flow was increased. Microvascular filtration pressure was the main determinant of fluid and protein transvascular filtration under these conditions.