We used plethysmograph techniques to measure or calculate the tissue and capillary forces and flows (capillary pressure, tissue and plasma oncotic pressure, transcapillary pressure drop, lymph flow, and interstitial pressure) in a dog hindpaw preparation in situ at three different venous pressures (PV). Since lymph was flowing from the preparation, an isovolumetric state represented an isofiltration state rather than the conventional isovolumetric or isogravimetric state. At an isofiltration capillary pressure of 12.8 mm Hg, lymph oncotic pressure averaged 3.8 mm Hg, plasma oncotic pressure averaged 20.9 mm Hg, and tissue pressure averaged -4.7 mm Hg (PV normal). The imbalance in transcapillary forces averaged 0.5 mm Hg and represented the lymph flow contribution (lymph flow/filtration coefficient) to maintainance of the normal isofiltration state. As isofiltration capillary pressure increased to 24.9 mm Hg, interstitial fluid volume increased by 1.7 ml/100g of tissue, tissue pressure rose by 4.6 mm Hg, lymph oncotic pressure fell by 2.2 mm Hg, and the transcapillary pressure drop increased to 5.6 mm Hg (PV = 20 mm Hg). At an isofiltration capillary pressure of 38.0 mm Hg, interstitial fluid volume increased by 17.5 ml/100g, interstitial pressure rose to +10 mm Hg, lymph oncotic pressure fell to 0.5 mm Hg, and the transcapillary pressure drop increased to 6.3 mm Hg (PV = 30 mm Hg). At moderate levels of PV elevation, the transcapillary pressure drop and increased tissue pressure provided 80% of the counterbalancing tissue force, each contributing approximately 40%. At higher venous pressures, the only tissue force that opposed filtration was an increase in tissue pressure.