Exposure of the apical surfaces of alveolar monolayers to acidic and alkaline solutions has been reported to have little influence on intracellular pH compared with basolateral challenges (Joseph D, Tirmizi O, Zhang X, Crandall ED, and Lubman RL. Am J Physiol Lung Cell Mol Physiol 282: L675-L683, 2002). We have used fluorescent pH indicators and a trifurcated optical bundle to determine whether the apical surfaces are less permeable to ionized buffers than the membranes that separate the vasculature from the tissues in intact rat lungs. In the first set of experiments, the air spaces were filled with perfusate containing FITC-dextran (mol wt 60000) or 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Air space pH fell progressively from 7.4 to 6.61 +/- 0.03 (mean +/- SE, n = 11, air space buffers at 10 mM). Perfusion for 2 min with 2 mM NH4Cl increased air space pH by 0.142 +/- 0.019 unit, without a subsequent acidic overshoot. Infusions of NaHCO3 and sodium acetate reduced pH without a subsequent alkaline overshoot. In the second set of experiments, cellular pH was monitored in air-filled lungs after perfusion with BCECFAM. Injections of NH4Cl caused a biphasic response, with initial alkalinization of the cellular compartment followed by acidification after the NH4Cl was washed from the lungs. Subsequent return of pH to normal was slowed by infusions of 1.0 mM dimethyl amiloride. These studies suggest that lung cells are protected from air space acidification by the impermeability of the apical membranes to buffer ions and that the cells extrude excess H+ through basolateral Na+/H+ exchangers.