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3-Nitropropionic acid (3NP), an irreversible inhibitor of succinate dehydrogenase, induces both rapid necrotic and slow apoptotic death in rat hippocampal neurons. Low levels of extracellular glutamate (10 μM) shift the 3NP-induced cell death mechanism to necrosis, while NMDA receptor blockade results in predominantly apoptotic death. In this study, we examined the 3NP-induced alterations in free cytosolic and mitochondrial calcium levels, ATP levels, mitochondrial membrane potential, and calpain and caspase activity, under conditions resulting in the activation of apoptotic and necrotic pathways. In the presence of 10 μM glutamate, 3NP administration resulted in a massive elevation in [Ca2+]c and [Ca2+]m, decreased ATP, rapid mitochondrial membrane depolarization, and a rapid activation of calpain but not caspase activity. In the presence of the NMDA receptor antagonist MK-801, 3NP did not induce a significant elevation of [Ca2+]c within the 24 h time period examined, nor increase [Ca2+]m within 1 h. ATP was maintained at control levels during the first hour of treatment, but declined 64% by 16 h. Calpain and caspase activity were first evident at 24 h following 3NP administration. 3NP treatment alone resulted in a more rapid decline in ATP, more rapid calpain activation (within 8 h), and elevated [Ca2+]m as compared to the results obtained with added MK-801. Together, the results demonstrate that 3NP-induced necrotic neuron death is associated with a massive calcium influx through NMDA receptors, resulting in mitochondrial depolarization and calpain activation; while 3NP-induced apoptotic neuron death is not associated with significant elevations in [Ca2+]c, nor with early changes in [Ca2+]m, mitochondrial membrane potential, ATP levels, or calpain activity.