Using an isolated, Ringer-perfused lung preparation, we measured values of the filtration coefficient and reflection coefficients of NaCl, mannitol, sucrose, raffinose, inulin, and albumin for the pulmonary capillary endothelium of adult rabbits. We found a filtration coefficient value of 1.00 × 10-3 cm3 sec-1 cm H2O-1. When this value is normalized for the estimated exchange area it is similar to other reported lung Kf values normalized in the same way. Using an osmotic transient technique we measured the reflection coefficients for the test molecules mentioned. NaCl, mannitol, sucrose, and raffinose all had small values (about 0.05) which were not significantly different from one another. Based on this we assume that using these molecules we measured the kinetics of osmotic flow across cell membranes and not through capillary endothelial pores (capillary σ = 0 for these molecules). Correcting inulin and albumin data for this "cell" effect, we estimate σinulin = 0.09 and σalbumin = 0.35 for adult pulmonary capillaries. These results are consistent with a two-pore model of the pulmonary capillary endothelium: (a) pores in the endothelial cell membranes having dimensions typical of cell membrane pores and contributing very little to fluid transport, and (b) pores through the endothelium having a radius of about 100 Å and through which virtually all induced fluid flux occurs. Our data, however, do not rule out other multipore models of the capillary endothelium which have been proposed.