The split-drop method has been used to determine filtration rate per unit surface area in the single pulmonary venule. In isolated perfused lungs of nine dogs, blood flow was stopped at different vascular pressures. By means of a double-micropuncture technique under stereomicroscopy, an oil drop was injected in a subpleural venule. The oil drop was then split with a solution of albumin (5.6 g/100 ml) in Ringer lactate. As the Ringer-albumin solution filtered, the distance between the menisci of the split oil drop (split-drop length) decreased. The split-drop geometry and the rate of change of split-drop length were recorded. The calculated venular filtration rate per unit surface area related linearly with vascular pressure (P less than 0.05). The slope of the line equaled venular hydraulic conductivity, which averaged 2.9 +/- 0.02 x 10(-7) ml/(cm2.s.cmH2O). Hydraulic conductivity is lower in lung than in systemic venules.