The mitochondrial genome codes for 13 proteins which are located in the respiratory chain. In postmortem brain of patients with Parkinson's disease, decreased activity of complex I of the respiratory chain could be demonstrated. Because seven subunits of complex I are coded by the mitochondrial genome, we analyzed the mitochondrial DNA of human postmortem substantia nigra, putamen, and frontal cortex by the Southern blot technique. No deletions of the mitochondrial genome could be demonstrated, thus indicating that either subunits which are encoded by the nuclear genome are decreased or enzyme activity is diminished by metabolites, toxins, or increase of Fe³⁺.