A Bax- and, apparently, mitochondria-dependent increase in superoxide (O2·−) and other reactive oxygen species (ROS) occurs in apoptotic superior cervical ganglion (SCG) and cerebellar granule (CG) neurons. Here we show that Bax also lies upstream of ROS produced in nonapoptotic neurons and present evidence that caspases partially mediate the pro-oxidant effect of Bax. We used the O2·−-sensitive dye MitoSOX to monitor O2·−in neurons expressing different levels of Bax and mitochondrial superoxide dismutase (SOD2). Basal and apoptotic O2·−levels in both SCG and CG neurons were reduced in SOD2 wild-type (WT) cells having lower Bax concentrations. Apoptotic and nonapoptotic neurons from Bax-WT/SOD2-null but not Bax-null/SOD2-null mice had increased O2·−levels. A caspase inhibitor inhibited O2·−in both apoptotic and nonapoptotic SCG neurons. O2·−production increased when WT, but not Bax-null, SCG neurons were permeabilized and treated with active caspase 3. There was no apoptosis and little increase in O2·−in SCG neurons from caspase 3-null mice exposed to an apoptotic stimulus. O2·−levels in nonapoptotic caspase 3-null SCG neurons were lower than in WT cells but not as low as in caspase inhibitor-treated cells. These data indicate that Bax lies upstream of most O2·−produced in neurons, that caspase 3 is required for increased O2·−production during neuronal apoptosis, that caspase 3 is partially involved in O2·−production in nonapoptotic neurons, and that other caspases may also be involved in Bax-dependent O2·−production in nonapoptotic cells.