Poly(ADP-ribose) polymerase-1 (PARP-1) is a nuclear enzyme activated by binding to DNA breaks, which causes PARP-1 automodification. PARP-1 activation is required for regulating various cellular processes, including DNA repair and cell death induction. PARP-1 involved in these regulations is localized in the nucleoplasm, but approximately 40% of PARP-1 can be found in the nucleolus. Previously, we have reported that nucleolar PARP-1 is delocalized to the nucleoplasm in cells exposed to DNA-damaging agents. However, the functional roles of this delocalization in cellular response to DNA damage is not well understood, since this approach simultaneously induces the delocalization of PARP-1 and its automodification. We therefore devised an approach for separating these processes. Unmodified PARP-1 was first delocalized from the nucleolus using camptothecin. Then, PARP-1 was activated by exposure of cells to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). In contrast to treatment with MNNG alone, delocalization of PARP-1 by CPT, prior to its activation by MNNG, induced extensive automodification of PARP-1. DNA repair activity and consumption of intracellular NAD(+) were not affected by this activation. On the other hand, activation led to an increased formation of apoptotic cells, and this effect was suppressed by inhibition of PARP-1 activity. These results suggest that delocalization of PARP-1 from the nucleolus to the nucleoplasm sensitizes cells to DNA damage-induced apoptosis. As it has been suggested that the nucleolus has a role in stress sensing, nucleolar PARP-1 could participate in a process involved in nucleolus-mediated stress sensing.