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The relationship is investigated between superoxide levels in single cultured rat cerebellar granule neurons exposed continuously to glutamate in low KCl medium and the deregulation of cytoplasmic Ca 2+ . Cells that maintain a regulated cytoplasmic-free Ca 2+ and mitochondrial polarization in the presence of glutamate show no increase in superoxide levels until the onset of cytoplasmic Ca 2+ deregulation. Oxidative stress of mitochondrial origin is readily detectable, as the inhibitors rotenone and antimycin A markedly increase superoxide levels with no effect on cytoplasmic-free Ca 2+ . The potent cell-permeant superoxide dismutase/catalase mimetic manganese tetrakis (N-ethylpyridinium-2yl) porphyrin, MnTE-PyP, abolishes the deregulation-related increase in superoxide but has no effect on deregulation itself. A combination of catalase with the free radical scavenger 4-hydroxy-TEMPO also fails to reduce deregulation. Following the loss of Ca 2+ homeostasis nuclei undergo condensation; this morphological change is not inhibited by MnTE-PyP and cannot account for the increased ethidium fluorescence. Phospholipase A 2 inhibitors decrease the deregulation-related increase in superoxide without protecting against deregulation. In conclusion, our study indicates that deregulation is not caused by NMDA receptor-mediated oxidative stress as NMDA receptor activation does not increase superoxide levels until the onset of deregulation. Furthermore, the majority of superoxide is produced in the cytoplasm rather than in mitochondria.