Neuronal cells injured by ischemia and reperfusion to a certain extent are committed to death in necrotic or apoptotic form. Necrosis is induced by gross ATP depletion or 'energy crisis' of the cell, whereas apoptosis is induced by a mechanism still to be defined in detail. Here, we investigated this mechanism by focusing on a DNA damage-sensor, poly(ADP-ribose) polymerase-1 (PARP-1). A 2-h oxygen and glucose deprivation (OGD) followed by reoxygenation (Reox) induced apoptosis, rather than necrosis, in rat cortical neurons. During the Reox, PARP-1 was much activated and autopoly(ADP-ribosyl)ated, consuming the substrate, NAD+. Induction of apoptosis by OGD/Reox was suppressed by overexpression of Bcl-2, indicating mitochondrial impairment in this induction process. Mitochondrial permeability transition (MPT), or membrane depolarization, and a release of proapoptotic proteins, i.e. cytochrome c, apoptosis-inducing factor and endonuclease G, from mitochondria were observed during the Reox. These apoptotic changes of mitochondria and the nucleus were attenuated by PARP-1 inhibitors, 1,5-dihydroxyisoquinoline and benzamide, and also by small interfering RNA specific for PARP-1. These results indicated that PARP-1 plays a principal role in inducing mitochondrial impairment that ultimately leads to apoptosis of neurons after cerebral ischemia.