The poly(ADP-ribose) polymerase (PARP) family of nuclear enzymes is involved in the detection and signaling of single strand breaks induced either directly by ionizing radiation or indirectly by the sequential action of various DNA repair proteins. Therefore, PARP plays an important role in maintaining genome stability. Because PARP proteins contain two zinc finger motifs, these enzymes can be targets for reactive nitrogen oxide intermediates (RNOS) generated as a result of nitric oxide (NO) biosynthesis in an aerobic environment. The effects of RNOS on the activity of purified PARP were examined using donor compounds. Both NO and nitroxyl (HNO) donors were found to be inhibitory in a similar time and concentration manner, indicating that PARP activity can be modified under both nitrosative and oxidative conditions. Moreover, these RNOS donors elicited comparable PARP inhibition in Sf21 insect cell extract and intact human MCF-7 cancer cells. The concentrations of donor required for 90% inhibition of PARP activity produce RNOS at a similar magnitude to those generated in the cellular microenvironment of activated leukocytes, suggesting that cellular scavenging of RNOS may not be protective against PARP modification and that inhibition of PARP may be significant under inflammatory conditions.