We measured the hydraulic conductivity of the visceral pleura in seven mongrel dogs in situ. The left chest was opened at the seventh intercostal space. A hemispherical capsule, filled with physiological saline, was attached to the visceral pleura of the left lower lobe by the negative pressure in the plate, using a vacuum pump. Transpleural fluid flow (V) was measured at different intracapsular pressures (delta P). The hydraulic conductivity was calculated from the relation between fluid flow and intracapsular pressure, i.e., the slope of the linear regression line. The hydraulic conductivity was 1.49 +/- 0.68 (mean +/- SD) nL.min-1 x cmH2O-2. Our values were smaller than those of former reports obtained in vivo. It is suggested that the dynamics mechanisms of plural effusion may be clarified by studies using our method.