To better understand the mechanism(s) underlying nitric oxide (· NO)-mediated toxicity, in the presence and absence of concomitant oxidant exposure, postmitotic terminally differentiated NT2N cells, which are incapable of producing · NO, were exposed to PAPA-NONOate (PAPA/NO) and 3-morpholinosydnonimine (SIN-1). Exposure to SIN-1, which generated peroxynitrite in the range of 25-750 nM/min, produced a concentration- and time-dependent delayed cell death. In contrast, a critical threshold concentration (>440 nM/min) was required for · NO to produce significant cell injury. Examination of cells by electron microscopy shows a largely necrotic injury after peroxynitrite exposure but mainly apoptotic-like morphology after · NO exposure. Cellular levels of reduced thiols correlated with cell death, and pretreatment with N-acetylcysteine (NAC) fully protected from cell death in either PAPA/NO or SIN-1 exposure. NAC given within the first 3 h posttreatment further delayed cell death and increased the intracellular thiol level in SIN-1 but not · NO-exposed cells. Cell injury from · NO was independent of cGMP, caspases, and superoxide or peroxynitrite formation. Overall, exposure of non-· NO-producing cells to · NO or peroxynitrite results in delayed cell death, which, although occurring by different mechanisms, appears to be mediated by the loss of intracellular redox balance.