We examined the effects of reactive oxygen-nitrogen intermediates on chloride (Cl⁻) currents across murine tracheal epithelial (MTE) cells isolated from CD-1 mice. MTE cells were cultured on permeable supports until they formed water-tight monolayers with transepithelial resistances (R_t) > 500 Ω/cm² and then were mounted in Ussing chambers. Baseline short-circuit current (I_SC) values, prior to and following the addition of 10 μM amiloride (an inhibitor of sodium-transport pathways) into the apical side, were 65 ± 1.9 μA/cm² and 7.6 ± 0.51 μA/cm², respectively (X ± 1 SE, n = 32). The addition of 3-morpholinosydnominine (SIN-1, 1 mM), which generates both superoxide and nitric oxide anions, to amiloride-treated monolayers resulted in a transient increase of I_SC to a peak value of 35 ± 1.3 μA/cm² (X ± SE, n = 14) within the next 30–60 min. After this, the I_SC decreased gradually and returned to its pre-SIN-1 value. These changes were blocked by glibenclamide (200 μM), an inhibitor of cystic fibrosis transmembrane regulator, or reduced by glutathione (GSH, 5 mM), a scavenger of peroxynitrite. Forskolin (10 μM) augmented the SIN-1 effect when added at the peak of the SIN-1 response but not when I_SC had returned to its baseline value. Perfusion of MTE cells with SIN-1 also increased whole cell Cl⁻ currents 4-fold and the open probability of CFTR-type single-channel currents from 0.041 to 0.92 in a transient fashion. Decomposed SIN-1, but not pure SIN-1c (the stable decomposition product of SIN-1), also increased I_SC with an EC₅₀ of 5 μM. Electrospray mass spectroscopy revealed several unique and uncharacterized compounds formed during the decomposition of SIN-1 as well as the reaction of SIN-1c with peroxynitrite. Formation of these compounds was inhibited by GSH. We conclude that compounds formed by the reaction of peroxynitrite with by-products of SIN-1, rather than reactive oxygen-nitrogen species per se, were responsible for the modulation of Cl⁻ secretion across primary cultures of MTE cells.