Peroxynitrite (ONOO-) is a potent oxidizing agent that initiates lipid peroxidation and sulfhydryl oxidation and may be responsible for a portion of the cytotoxicity attributed to superoxide anion (O2-). We quantified the extent to which ONOO-, xanthine plus xanthine oxidase (XO) and hydrogen peroxide (H2O2), decreased sodium (Na+) uptake into membrane vesicles derived from colonic cells of dexamethasone-treated rats. Carrier-free 22Na+ uptake into vesicles was measured in the presence of an inside negative membrane potential, produced by the addition of the potassium ionophore valinomycin (10 μM) after removal of all external potassium by cation exchange chromatography. Preincubation of vesicles with either 100 μM or 1 mM ONOO- for 30 s decreased the amiloride-blockable fraction of Na+ uptake by 27 ± 7% and 65 ± 2%, respectively (X ± S.E.; n ≥ 5; P < 0.05 from control). However, the amiloride-insensitive part of Na+ uptake was not affected, indicating that there was no overt destruction of these vesicles by these ONOO- concentrations. Decomposed ONOO-, hydrogen peroxide (1 μM-10 mM), or xanthine (500 μM) plus XO (10-30 mU/ml), either in the absence or in the presence of 100 μM FeEDTA, did not decrease Na+ uptake. These data suggest that ONOO- is a potent injurious agent that can compromise Na+ uptake across epithelial cells possibly by damaging Na+ channels. © 1992.