Plasma oncotic pressure is considered to be an important factor in controlling lung water after hemorrhagic shock. However, it is the gradient between plasma and interstitial oncotic pressure which affects the pulmonary transvascular fluid filtration rate, Qf. Our objective was to determine the effect of decreasing plasma oncotic pressure, pip, on Qf, on interstitial oncotic pressure pii, and on the oncotic gradient. Chronic lung lymph fistulas were created in 16 sheep. Lymph flow, a reliable index of Qf, plasma and lymph (equal to interstitial) oncotic pressures, and vascular pressures were monitored in unanesthetized sheep, before and during hemorrhagic shock (50% blood volume), during resuscitation (3 hours), and during recovery (24 hours). Resuscitation was either with shed blood or lactated Ringer's solution in sufficient quantity to return left atrial pressure and cardiac output to baseline levels. During resuscitation with blood, lymph flow increased by 115%. The pip remained constant, while pii decreased, increasing the oncotic gradient. Crystalloid resuscitation produced on increase in lymph flow equal to that in the blood group at 120% over baseline; however, pip decreased by 50%, producing an oncotic gradient 4 mm Hg less than that of blood group. This was not reflected by a difference in Qf between the groups. During recovery Qf returned to baseline in the blood group and in most of the crystalloid group, as the oncotic gradient returned to baseline, despite a significant decrease in pip due to a compensatory decrease in pii. We conclude that during resuscitation Qf does not appear to be increased by a decrease in the oncotic gradient. During recovery a major decrease in pip can be compensated for rapidly by a decrease in pii, leading to no change in interstitial fluid content.