Nitric oxide (NO) shows an unusual divergence of action, being utilised both as a physiological signalling molecule, and as a toxic mediator. NO-mediated cellular injury may arise by a variety of mechanisms, including disruption of mitochondrial respiration, enzyme inhibition, lipid peroxidation and genetic mutation. Toxicity is largely mediated via intermediates such as N2O3 and peroxynitrite, arising from the reaction of NO with either molecular oxygen or reactive oxygen species. In general, such reactions become significant only when high concentrations of NO are generated by the induction of nitric oxide synthase.