The composition and depth of the airway surface liquid (ASL) are key parameters in airway physiology that are thought to be important in the pathophysiology of cystic fibrosis and other diseases of the airways. We reported novel fluorescent indicator and microscopy methods to measure [Na+], [Cl-], pH, and depth of the ASL in large airways (Jayaraman, S., Y. Song, L. Vetrivel, L. Shankar, and A.S. Verkman. 2001. J. Clin. Invest. 107:317-324.). Here we report a stripped-lung preparation to measure ASL composition and depth in small distal airways. Distal ASL was stained with ion- or pH-sensitive fluorescent indicators by infusion into mouse trachea of a perfluorocarbon suspension of the indicator. After stripping the pleura and limited microdissection of the lung parenchyma, airways were exposed for measurement of ASL [Na+], [Cl-], and pH by ratio imaging microscopy, and depth by confocal microscopy. The stripped-lung preparation was validated in stability and tissue viability studies. ASL [Na+] was 122 +/- 2 mM, [Cl-] was 123 +/- 4 mM and pH was 7.28 +/- 0.07, and not dependent on airway size (250-mum diameter), ENaC inhibition by amiloride, or CFTR inhibition by the thiazolidinone CFTRinh-172. ASL depth was 8-35 mum depending on airway size, substantially less than that in mouse trachea of approximately 55 mum, and not altered significantly by amiloride. These results establish a novel lung preparation and fluorescence approach to study distal airway physiology and provide the first data on the composition and depth of distal ASL.