Huntington's disease (HD) is a prototypical neurodegenerative disease characterized by selective loss of neurons in the basal ganglia. Although the gene defect has recently been identified, the mechanism by which it leads to neuronal degeneration remains unknown. We have hypothesized that a defect in oxidative phosphorylation may lead to slow, excitotoxic neuronal degeneration in this illness. Evidence for such a defect is reviewed here, including our recent studies using magnetic resonance imaging spectroscopy that show elevated lactate levels in the basal ganglia and cerebral cortex of patients with HD. If a defect in energy metabolism is responsible for neuronal degeneration in HD, it should be possible to mimic the neurodegenerative process with mitochondrial toxins. Our recent studies with 3-nitropropionic acid, an irreversible inhibitor of succinate dehydrogenase, show that it can produce striking similarities to the neuropathologic and neurochemical features of HD in both rodents and primates. If such a mechanism is indeed relevant to the pathogenesis of HD, then agents that can improve oxidative phosphorylation might prove to be efficacious. We found that both coenzyme Q10 and nicotinamide can ameliorate striatal lesions produced by mitochondrial toxins in vivo. Furthermore, they reduced elevated lactate concentrations when administered to patients with HD. This finding raises the possibility that such an approach might prove useful in trying to slow the neurodegenerative process.