Step additions of small hydrophilic solutes to the perfusate of isolated dog lungs produce weight changes (Vargas-Johnson). The product: (observed reflection coefficient) times (observed filtration coefficient) for the lung endothelial barrier is obtained from the initial weight slope per unit osmolality change. The observed filtration coefficient determined from the sustained constant weight slope per increment of pulmonary outflow pressure was 0.038 plus or minus 0.011 cm/s (assumed exchange area 500 cm2/g lung). Correction to average transcapillary pressure (32% upward) and for osmotic buffering by resident solute (25% upward) gave 0.063 plus or minus 0.018 cm/s. The observed reflection coefficients were (mean plus or minus 2 SE): sodium chloride, 0.30 plus or minus 0.11; urea, 0.30 plus or minus 0.12; glucose, 0.48 plus or minus 0.26; sucrose, 0.39 plus or minus 0.17; raffinose, 0.35 plus or minus 0.16; hypotonic response: interstitial solute, 0.26 plus or minus 0.10. The observed reflection coefficient of sodium chloride, corrected for the preceding filtration coefficient effects (65% downward) and for combined flow limitation and osmotic buffering effects (50% upward) was 0.27. Transcapillary gradients of small hydrophilic solutes produce substantial osmotic effects for short times (seconds). Passive exchanges and net fluxes across the endothelium of the lung capillaries have parameters similar to those of the endothelium of skeletal muscle.