Poly(adenosine 5 prime-diphosphoribose) synthetase (PARS) is a nuclear enzyme which, when activated by DNA strand breaks, adds up to 100 adenosine 5 prime-diphosphoribose (ADP-ribose) units to nuclear proteins such as histones and PARS itself. This activation can lead to cell death through depletion of beta-nicotinamide adenine dinucleotide (the source of ADP-ribose) and adenosine triphosphate. Nitric oxide (NO) stimulated ADP-ribosylation of PARS in rat brain. Benzamide and other derivatives, which inhibit PARS, blocked N-methyl-D-aspartate-and NO-mediated neurotoxicity with relative potencies paralleling their ability to inhibit PARS. Thus, NO appeared to elicit neurotoxicity by activating PARS.