The pulmonary lymph flow rate (QL) should be a function of the lymph vessels' resistance and the pressure gradient along the vessels. We attempted to study how these factors affect lymph flow. We assumed that the lymph system could be represented by a single pressure generated within the lung (PL) and a single resistance (RL). Thus, QL should be a function of the lymph vessel outflow pressure (Po): QL = (PL - Po)/RL. We cannulated tracheobronchial lymph vessels in eight anesthetized dogs and varied Po by raising the outflow end of the cannula. QL decreased linearly when we increased Po. We estimated RL as - delta Po/ delta QL and PL as the extrapolated Po at which QL = 0. At baseline PL = 7.7 +/- 2.7 (SD) cm H2O and RL = 0.36 +/- 0.25 cm H2O. min/microliters. After we increased capillary pressure to produce edema, PL and RL averaged 22.8 +/- 8.8 and 0.14 +/- 0.12, respectively. After we reduced the capillary pressure to baseline in the edematous lungs, PL and RL averaged 11.6 +/- 2.8 and 0.08 +/- 0.09, respectively. All changes in PL and RL were significant (P less than 0.05). These results show that (1) lymph flow rate depends upon lymph vessel outflow pressure, and (2) the QL vs. Po relationship is changed by edema. PL may be equal to the pressure causing lymph to flow and RL may equal the lymph vessel resistance.