A surface fluorescence method was developed to measure transalveolar transport of water, protons, and solutes in intact perfused lungs. Lungs from c57 mice were removed and perfused via the pulmonary artery (approximately 2 ml/min). The airspace was filled via the trachea with physiological saline containing a membrane-impermeant fluorescent indicator (FITC-dextran or aminonapthalene trisulfonic acid, ANTS). Because fluorescence is detected only near the lung surface due to light absorption by lung tissue, the surface fluorescence signal is directly proportional to indicator concentration. Confocal microscopy confirmed that the fluorescence signal arises from fluorophores in alveoli just beneath the pleural surface. Osmotic water permeability (Pf) was measured from the time course of intraalveolar FITC-dextran fluorescence in response to changes in perfusate osmolality. Transalveolar Pf was 0.017 +/- 0.001 cm/s at 23 degrees C, independent of the solute used to induce osmosis (sucrose, NaCl, urea), independent of osmotic gradient size and direction, weakly temperature dependent (Arrhenius activation energy 5.3 kcal/mol) and inhibited by HgCl2. Pf was not affected by cAMP activation but was decreased by 43% in lung exposed to hyperoxia for 5 d. Diffusional water permeability (Pd) and Pf were measured in the same lung from intraalveolar ANTS fluorescence, which increased by 1.8-fold upon addition of 50% D2O to the perfusate, Pd was 1.3 x 10(-5) cm/s at 23 degrees C. Transalveolar proton transport was measured from FITC-dextran fluorescence upon switching perfusate pH between 7.4 and 5.6; alveolar pH half-equilibrated in 1.9 and 1.0 min without and with HCO3-, respectively. These results indicate high transalveolar water permeability in mouse lung, implicating the involvement of molecular water channels, and establish a quantitative surface fluorescence method to measure water and solute permeabilities in intact lung.