A previous study from this laboratory demonstrated that lymph nodes can change the protein concentration and colloid osmotic pressure of lymph by transfer of protein-free fluid between the blood and lymph compartments. In that study a Starling force disequilibrium across the blood-lymph barrier caused fluid to transfer through the barrier in the direction required to establish equilibrium of Starling forces. In the present study we examined the effect of increased lymph node venous blood pressure on efferent lymph protein concentration and efferent lymph flow. We utilized an isolated dog popliteal lymph node preparation in which afferent lymph having various protein concentrations was perfused into the node at an average flow rate of 19.1 +/- 0.3 (SD) microliter/min. We compared steady-state values of prenodal and postnodal lymph flows and protein concentrations during various steady-state levels of lymph node venous blood pressure. When venous pressure was increased, the protein concentration of the efferent lymph invariably decreased and the efferent flow rate invariably increased. Measurements showed that an average of 96% of the change in lymph protein concentration was caused by transfer of protein-free fluid through the lymph node blood-lymph barrier. The results of this study indicate again that the lymph node functions as a fluid exchange chamber in which fluid is transferred between the blood and lymph compartments in the direction required to establish equilibrium of the Starling forces across the blood-lymph barrier.