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DNA is a primary site of damage during oxidative stress in the brain. DNA fragmentation occurs within minutes of induction of oxidative stress. This DNA fragmentation probably results from the attack of free radicals on DNA and from the activation of endonucleases. Oxidative stress was induced by intracerebroventricular injection of t-butylhydroperoxide. This results in a very rapid flux of t-butylhydroperoxide, which is cleared from the brain within minutes. This flux of t-butylhydroperoxide results in the formation of hydroxyl radical in the brain and probably in the nuclei of brain cells. Necrosis results from extensive DNA fragmentation caused by massive oxidative stress. Cresyl violet stained brain sections demonstrated necrosis in many brain regions. In addition, previous electron microscopy studies showed degradation of cellular nuclei caused by tBuOOH toxicity. Low doses of t-butylhydroperoxide can induce apoptosis, which is a delayed form of cell death. Apoptosis was found in brains stained to visualize apoptotic DNA fragments. Experiments performed in mice aged 2, 8 or 24 months will be discussed. We have also found that apoptosis and DNA fragmentation can be prevented by pretreating mice with the vitamin micotinamide. Nicotinamide is a precursor for NAD. DNA repair requires high levels of NAD in the nucleus for the activity of poly(ADP-ribose) polymerase. Oxidative stress in the brain produces both necrosis and apoptosis, probably as the result of DNA fragmentation. Senescence is associated with an increase in the production of DNA fragments during brain oxidative stress, which probably leads to more necrosis and apoptosis than in younger mice.