Active sodium (Na+) transport by alveolar type II (ATII) cells plays an important role in limiting the volume of alveolar fluid. Reactive oxygen and nitrogen species, released in the epithelial lining fluid by activated inflammatory cells or present in inspired gases, may damage Na+ transporters and decrease fluid reabsorption. To test this hypothesis we exposed ATII cells to xanthine and xanthine oxidase (1 or 10 mU/ml), or to boluses of peroxynitrite (0.1-1 mM final concentration) for 15 min and measured 1) cellular oxygen consumption (V̇O2); 2) amiloride-inhibitable 22Na+ uptake, as an index of Na+ movement through apically located Na+ channels; and 3) ouabain-sensitive 86Rb+ uptake, as an index of the activity of the basolaterally located Na+-K+-ATPase. After exposure of ATII cells to 0.5 or 1 mM peroxynitrite, amiloride-inhibitable 22Na+ uptake decreased to 68 ± 7 and 56 ± 11 of their control values, respectively (means ± SE; n ≥ 6). Exposure to 0.5 mM peroxynitrite decreased ATII cell V̇O2 from 76 ± 6 to 25 ± 5 μM · h-1 · 106 cells-1 (means ± SE; n = 5). Cell viability and ouabain-sensitive 86Rb+ uptake remained at control levels for either peroxynitrite concentration. Exposure of ATII cells to 10 mU/ml xanthine oxidase decreased their V̇O2 from 94 ± 8 to 63 ± 6 (means ± SE; n = 5), but did not alter amiloride-inhibitable 22Na+ uptake. These findings indicate that physiological concentrations of peroxynitrite, but not of reactive oxygen species, decrease ATII cell Na+ transport by damaging apically located amiloride-sensitive Na+ channels.