The measurement of pulmonary vascular pressure and plasma oncotic pressure (πp) is reported to be useful for predicting excess fluid transport across the pulmonary microcirculation in the critically ill. However, interpretation of change in these forces must take into consideration the compensatory changes in the other Starling forces, particularly interstitial oncotic pressure (πi) and the permeability characteristics of the membrane. We calculated pulmonary microvascular pressure Pmv, πp, and πi in unanesthetized sheep with lung lymph fistulae. We altered Pmv by fluid loading and πp by producing a severe hypoproteinemia after hemorrhagic shock and altered the membrane with endotoxin. We compared these with changes in the pulmonary transvascular fluid filtration rate (&OV0422;f). We found Pmv correlated very well with &OV0422;fduring volume loading and during the post-shock hypoproteinemic state, πi decreased in response to decreases in πp, producing a relatively constant oncotic gradient (πp-πi). πp then correlated very poorly with &OV0422;fwhen hypoproteinemia was present. There was essentially no correlation between any of the pressures and &OV0422;fafter the severe permeability change produced by endotoxin.