In mammalian cells, the nucleosome-binding protein HMGN1 (high mobility group N1) affects the structure and function of chromatin and plays a role in repair of damaged DNA. HMGN1 affects the interaction of DNA repair factors with chromatin and their access to damaged DNA; however, not all of the repair factors affected have been identified. Here, we report that HMGN1 affects the self-poly(ADP-ribosyl)ation (i.e., PARylation) of poly(ADP-ribose) polymerase-1 (PARP-1), a multifunctional and abundant nuclear enzyme known to recognize DNA lesions and promote chromatin remodeling, DNA repair, and other nucleic acid transactions. The catalytic activity of PARP-1 is activated by DNA with a strand break, and this results in self-PARylation and PARylation of other chromatin proteins. Using cells obtained from Hmgn1(-/-) and Hmgn1(+/+) littermate mice, we find that in untreated cells, loss of HMGN1 protein reduces PARP-1 self-PARylation. A similar result was obtained after MMS treatment of these cells. In imaging experiments after low energy laser-induced DNA damage, less PARylation at lesion sites was observed in Hmgn1(-/-) than in Hmgn1(+/+) cells. The HMGN1 regulation of PARP-1 activity could be mediated by direct protein-protein interaction as HMGN1 and PARP-1 were found to interact in binding assays. Purified HMGN1 was able to stimulate self-PARylation of purified PARP-1, and in experiments with cell extracts, self-PARylation was greater in Hmgn1(+/+) than in Hmgn1(-/-) extract. The results suggest a regulatory role for HMGN1 in PARP-1 activation.