U937 human myeloid leukemia cells respond to mild treatment with hydrogen peroxide and hyperthermia by undergoing apoptosis, an active mode of cell suicide. Higher concentrations of hydrogen peroxide, or longer incubation at the hyperthermic temperature, change the mode of cell death from apoptosis to the passive necrosis. Stress treatments cause a severe drop in the intracellular NAD concentration. 3-Aminobenzamide (3-ABA), a specific inhibitor of poly(ADP-ribosyl) polymerase (PARP), a nuclear enzyme which is activated by breaks in DNA to catabolize intracellular NAD, is capable of relieving such a drop. This suggests that breaks in DNA have been induced by both oxidative stress and heat shock, thereby activating PARP. Upon stress, NAD concentration has a first initial sharp drop; then, for mild stress treatments, it recovers, just when apoptosis begins to be detectable (8 h of recovery). At 20 h, when the apoptotic ladder-like pattern of DNA is visible, NAD concentration has dropped again, probably because of a second PARP activation due to the extensive DNA degradation that accompanies apoptosis. The presence of 3-ABA, concomitantly with the preservation of the intracellular NAD content, reduces the extent of apoptosis upon oxidative stress and strongly enhances cell survival, thus suggesting a role for PARP in triggering stress-induced apoptosis. All apoptotic U937 cells have a reduced NAD content, independently of the inducing agent; however, upon treatments which do not cause immediate DNA breaks, the drop in NAD concentration occurs only after the apoptotic ladder is detectable and can be ascribed to the activation of PARP by the free ends of DNA formed during the endonucleolytic degradation. Moreover, in these instances the inhibition of PARP, although effective in blocking the drop in NAD concentration, has no effect on apoptosis, thus being only circumstantial.