The recent discovery of mitochondrial complex I deficiency in the substantia nigra of patients with idiopathic Parkinson's disease has provided new understanding into the possible mechanisms that may underlie this neurodegenerative disorder. The biochemical defect is identical to that induced in humans, primates and mice exposed to the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. We have studied mitochondrial respiratory chain function in various brain regions, in skeletal muscle and in blood platelets from patients with idiopathic Parkinson's disease and from matched controls. We provide evidence suggesting that the complex I deficiency in Parkinson's disease is limited to the brain and that this defect is specific for the substantia nigra. The tissue specificity of the complex I deficiency in Parkinson's disease and its localization to the substantia nigra support the proposition that complex I deficiency may be directly involved in the cause of dopaminergic cell death in Parkinson's disease. An understanding of the molecular basis of this biochemical defect will provide valuable insight into the cause of Parkinson's disease. Our findings of normal mitochondrial function in platelet homogenates suggests that this tissue cannot be used to develop a 'diagnostic test' for Parkinson's disease.