3-Morpholinosyndnomine (SIN-1) has been reported to be a peroxynitrite (OONO(-)) donor because it produces both nitric oxide (NO) and superoxide (O(2)(-).) upon decomposition in aqueous solution. However, SIN-1 can decompose to primarily NO in the presence of electron acceptors, including those found in biological tissues, making it necessary to determine the release product(s) formed in any given biological system. In a mixed cortical cell culture system, SIN-1 caused a concentration-dependent increase in cortical cell injury with a parallel increase in the release of cellular proteins containing 3-nitrotyrosine into the culture medium. The increase in 3-nitrotyrosine immunoreactivity, a footprint of OONO(-) production, was specific for SIN-1 as exposure to neurotoxic concentrations of an NO donor (Z)-1-[2-aminoethyl)-N-(2-ammonioethyl) aminodiazen-1-ium-1,2-diolate (DETA/NO), or NMDA did not result in the nitration of protein tyrosine residues. Both SIN-1-induced injury and 3-nitrotyrosine staining were prevented by the addition of either 5,10,15,20-Tetrakis (4-sulfonatophenyl) prophyrinato iron (III) [FeTPPS], an OONO(-) decomposition catalyst, or uric acid, an OONO(-) scavenger. Removal of NO alone was sufficient to inhibit the formation of OONO(-) from SIN-1 as well as its cytotoxicity. Removal of O(2)(-). and the subsequently formed H(2)O(2) by superoxide dismutase (SOD) plus catalase likewise prevented the nitration of protein-bound tyrosine but actually enhanced the cytotoxicity of SIN-1, indicating that cortical cells can cope with the oxidative but not the nitrosative stress generated. Finally, neural injury induced by SIN-1 in unadulterated cortical cells was prevented by antagonism of AMPA/kainate receptors, while blockade of the NMDA receptor was without effect. In contrast, activation of both NMDA and non-NMDA receptors contributed to the SIN-1-mediated neurotoxicity when cultures were exposed in the presence of SOD plus catalase. Thus, whether SIN-1 initiates neural cell death in an OONO(-)-dependent or -independent manner is determined by the antioxidant status of the cells. Further, the mode of excitotoxicity by which injury progresses is determined by the NO-related species generated.