Different abnormalities in mitochondrial electron transport chain activity have been demonstrated in muscle and other tissues of patients with idiopathic Parkinson's disease (PD). We studied eight Spanish patients with PD to evaluate the functional activity of the electron transport chain in muscle mitochondria from patients of this country. We found lower complex I activity (nmol.min-1.mg-1) in patients (245.8 +/- 42.8) than in controls (331.6 +/- 60.1) (p = 0.004) and lower complex IV activity in patients (46.1 +/- 9) than in controls (144.1 +/- 42.3) (p = 0.00001). Complex V activity was also decreased in two patients and complex II and III activities were normal in all of them. Although these results strongly suggest an alteration in mitochondrial DNA in PD, the various electron transport chain defects in different tissues seem to be nonspecific.