[en] In situ and isolated fluid-filled rabbit lungs were used to study the transport of indicators between the air space and vascular compartments. These indicators were placed in either the perfusate or air spaces and samples were collected from the perfusate at intervals during a 1-h perfusion period. At the end of the hour, fluid was pumped out of the air space compartment into serial tubes and indicator concentrations were determined in both the air space and perfusion fluids. One hour after introducing the indicators into the air space, the relative decreases in solute concentration were (arranged from the greatest to the least decline): [¹⁴C]urea greater than ³⁶Cl^- = ¹²⁵I^- greater than ²²Na⁺ greater than [3H]mannitol. The relative rates at which the indicators appeared in the perfusate were similar. When the indicators were placed in the perfusate, a similar relationship was observed in the increase in air space concentrations, but the loss of ²²Na⁺ from the perfusate was similar to those of ³⁶Cl^- and ¹²⁵I^-. Losses of all indicators from the perfusate were two or more times those from the air spaces, and although the loss of [3H]mannitol from the perfusate was similar to that of ²²Na⁺ for about 30 min, subsequent loss was much slower. Very little ¹²⁵I-albumin traversed the tissue barrier, and the small changes in the concentrations of ¹²⁵I-albumin in the air spaces suggested that little fluid movement had occurred. These studies suggest that the epithelium is less permeable to solutes than the endothelium and permits passage of anions at a faster rate than ²²Na⁺