The contribution of pulmonary extra-alveolar vessels toward edema development was investigated in isolated dog lungs. In 27 lungs the intra-alveolar vessels were shut by alveolar pressure exceeding both arterial and venous pressures. Fluid transudation from extra-alveolar vessels into the interstitial space was estimated both by lung weight gain and serial biopsies examined histologically. Interstitial fluid cuffing was consistently seen around arteries, veins, and airways when the lungs had gained from 4% to 6% of their original weight. The rate of fluid accumulation was related to vascular pressure but not to alveolar pressure. Evans blue dye mixed in the blood was used to test vessel permeability to water. Measurements of the internal diameters of permeable vessels demonstrated a wider size range for veins than for arteries, the upper limits being 750 FX; and 200 [i, respectively. Static experiments, in which the whole pulmonary vascular tree was filled, were compared with those in which the intra-alveolar vessels were shut. For the same vascular pressures, the extra-alveolar vessels accounted for 63% of the weight gain when the whole vascular tree was filled.