Treatment of cells with DNA damaging agents results in a dose dependent decrease in NAD+ and ATP pool sizes. The decrease in NAD+ is associated with the activation of poly(ADP-ribose) polymerase and the decrease in ATP is consequent to the fall in NAD1+. Depletion of both NAD+ and ATP can be blocked or retarded by inhibitors of poly(ADP-ribose) polymerase. Both the stimulation of poly(ADP-ribose) synthesis and the effect of enzyme inhibitors have been confirmed in intact cells by using enzymatic cycling techniques to measure the disappearance of NAD+ and high pressure liquid chromatography (HPLC) to measure fluctuations in polymer levels. As a consequence of the depletion of NAD+ and ATP pools, cells exhibit a marked impairment in their ability to conduct all energy dependent functions. Thus cells treated with high levels of DNA damaging agents exhibit severe suppression of DNA replication and repair, RNA synthesis and protein synthesis. The use of inhibitors of poly-(ADP-ribose) polymerase to prevent the depletion of NAD+ and ATP partially restores the cells’ ability to conduct DNA, RNA, and protein synthesis. This preservation of the NAD+ and ATP pools accounts for the recent observations that inhibitors of poly(ADP-ribose) stimulate the level of DNA repair synthesis in cells treated with high levels of DNA damaging agents. We have also examined cells from patients with several of the disorders of DNA repair and have found that cells from patients with Fanconi’s anemia have lower than normal NAD+ levels. When cells from these patients are treated with DNA damaging agents their NAD+ pools are depleted to levels that are lower than those which occur in cells from normal donors. An impaired ability to maintain nucleotide pools and energy dependent functions may contribute to the decreased survival that Fanconi’s anemia cells exhibit following DNA damage. Thus in eukaryotes, high levels of DNA damage activate poly(ADP-ribose) polymerase to a degree that depletes cellular NAD+ levels and subsequently depletes ATP levels causing a decrease in energy dependent functions which can consequently lead to cell death before DNA repair can be accomplished. Such a suicide mechanism may be of benefit to an organism where it would be preferrable for cells with severely damaged DNA to die rather than risk repair with a high level of infidelity.