Defects in mitochondrial oxidative metabolism, in particular decreased activity of cytochrome c oxidase, have been demonstrated in Alzheimer’s disease, and after the expression of the amyloid precursor protein (APP) in cultured cells, suggesting that mitochondria might be involved in β-amyloid toxicity. Recent evidence suggests that the proteolysis of APP to generate β-amyloid is at least in part intracellular, preceding the deposition of extracellular fibrils. We have therefore investigated the effect of incubation of isolated rat brain mitochondria with the β-amyloid fragment 25–35 (100 μM) on the activities of the mitochondrial respiratory chain complexes I, II–III, IV (cytochrome c oxidase) and citrate synthase. The peptide caused a rapid, dose-dependent decrease in the activity of complex IV, while it had no effect on the activities on any of the other enzymes tested. The reverse sequence peptide (35–25) had no effect on any of the activities measured. We conclude that inhibition of mitochondrial complex IV might be a contributing factor to the pathogenesis of Alzheimer’s disease.