Present study demonstrated that fibrillar beta-amyloid peptide (fAbeta1-42) induced ATP release, which in turn activated NADPH oxidase via the P2X7 receptor (P2X7R). Reactive oxygen species (ROS) production in fAbeta1-42- treated microglia appeared to require Ca2+ influx from extracellular sources, because ROS generation was abolished to control levels in the absence of extracellular Ca2+. Considering previous observation of superoxide generation by Ca2+ influx through P2X7R in microglia, we hypothesized that ROS production in fAbeta-stimulated microglia might be mediated by ATP released from the microglia. We therefore examined whether fAbeta1-42-induced Ca2+ influx was mediated through P2X7R activation. In serial experiments, we found that microglial pretreatment with the P2X7R antagonists Pyridoxal-phosphate-6-azophenyl-2',4'- disulfonate (100 microM) or oxidized ATP (100 microM) inhibited fAbeta-induced Ca2+ influx and reduced ROS generation to basal levels. Furthermore, ATP efflux from fAbeta1-42- stimulated microglia was observed, and apyrase treatment decreased the generation of ROS. These findings provide conclusive evidence that fAbeta-stimulated ROS generation in microglial cells is regulated by ATP released from the microglia in an autocrine manner.