Hydraulic conductivity of the visceral pleura was measured in situ in anesthetized dogs. There were two groups: control (n = 7), and edema (n = 5). The 7th intercostal space of the left thorax was opened. In each group, a hemispherical capsule, filled with physiological salt solution, was attached to the visceral pleura of left lobe by negative pressure made with a vacuum pump. In the edema group, pulmonary venous pressure was increased by ligation of the pulmonary vein. The transpleural fluid flow (V) was measured at different intracapsular pressures (delta P). The hydraulic conductivity was calculated from the relation between the fluid flow rate (v) and the intracapsular pressure, i.e., the slope of the linear regression line. The hydraulic conductivities in the control and edema groups were 1.49 +/- 0.68 and 3.19 +/- 1.13 nL.min-1.cmH2O-1.cm-2, respectively. We conclude that the pleural tissue may play an important role in hydraulic conductivity of the visceral pleura when pulmonary venous pressure is high.