Striatal cell death in Huntington's Disease (HD) may involve mitochondrial defects, NMDA-mediated excitotoxicity, and activation of death effector proteases such as caspases and calpain. However, the precise contribution of mitochondrial defects in the activation of these proteases in HD is unknown. Here, we addressed this question by studying the mechanism of striatal cell death in rat models of HD using the mitochondrial complex II inhibitor 3-nitropropionic acid (3-NP). The neurotoxin was either given by intraperitoneal injections (acute model) or over 5 d by constant systemic infusion using osmotic pumps (chronic model) to produce either transient or sustained mitochondrial deficits. Caspase-9 activation preceded neurodegeneration in both cases. However, caspase-8 and caspase-3 were activated in the acute model, but not in the chronic model, showing that 3-NP does not require activation of these caspases to produce striatal degeneration. In contrast, activation of calpain was specifically detected in the striatum in both models and this was associated with a calpain-dependent cleavage of huntingtin. Finally, in the chronic model, which mimics a steady blockade of complex II activity reminiscent of HD, selective calpain inhibition prevented the abnormal calpain-dependent processing of huntingtin, reduced the size of the striatal lesions, and almost completely abolished the 3-NP-induced DNA fragmentation in striatal cells. The present results demonstrate that calpain is a predominant effector of striatal cell death associated with mitochondrial defects $in\ vivo$. This suggests that calpain may play an important role in HD pathogenesis and could be a potential therapeutic target to slow disease progression.