Acute ammonia toxicity is mediated by excessive activation of NMDA receptors. Activation of NMDA receptors leads to activation of poly(ADP-ribose) polymerase (PARP) which mediates NMDA excitotoxicity. PARP is activated following DNA damage and may lead to cell death via NAD+and ATP depletion. The aim of the present work was to assess whether acute ammonia intoxication in vivo leads to increased PARP in brain cells nuclei and to altered NAD+and superoxide metabolism and the contribution of NMDA receptors to these alterations. Acute ammonia intoxication increases PARP content twofold in brain cells nuclei. NAD+content decreased by 55% in rats injected with ammonia. This was not due to decreased NAD+synthetase nor increased NAD+hydrolase activities and would be due to increased NAD+consumption by PARP. Superoxide radical formation increased by 75% in nuclei of brains of rats injected with ammonia, that also induced protein nitrotyrosylation and DNA damage. Blocking NMDA receptors prevented ammonia-induced PARP, superoxide and nitrotyrosylation increase, DNA damage and NAD+decrease. These results show that acute ammonia intoxication in vivo leads to activation of NMDA receptors, leading to increased superoxide formation and PARP content and depletion of NAD+in brain cells nuclei that contribute to ammonia toxicity.