Recently, it was proposed that zymosan, a nonbacterial agent, causes cellular injury by inducing the production of peroxynitrite and consequent poly-(ADP-ribose) synthetase (PARS activation). Here we investigated whether In vivo N-acetylcysteine treatment inhibits cellular injury in macrophages collected from rats subjected to zymosan-induced shock. Macrophages harvested from the peritoneal cavity exhibited a significant production of peroxynitrite, as measured by the oxidation of the fluorescent dye dihydrorhodamine 123, and by nitrotyrosine. Furthermore, zymosan-induced shock caused a suppression of macrophage mitochondrial respiration, DNA strand breakage, and reduction of cellular levels of NAD+. In vivo treatment with N-acetylcysteine (40, 20, and 10 mg/kg, intraperitoneally, 1 and 6 h after zymosan) significantly reduced in a dose-dependent manner peroxynitrite formation and prevented the appearance of DNA damage, the decrease in mitochondrial respiration, and the loss of cellular levels of NAD+. Our study supports the view that the antioxidant and anti-inflammatory effect of A/-acetylcysteine is also correlated with the inhibition of peroxynitrite production. In conclusion, N-acetylcysteine may be a novel pharmacological approach to prevent cell injury in inflammation.