Biochemical and mitochondrial DNA analyses were performed in post-mortem brain tissue from seven patients with dementia of Alzheimer's type and age- and sex-matched controls. We analysed all complexes of the respiratory chain in four regions, i.e. temporal, parietal, entorhinal cortex and hippocampus. Although enzymes representing complex II, III and IV were reduced in activity, succinate cytochrome c reductase was significantly reduced only in the parietal and temporal cortex. However, Southern blot analyses with two restriction enzymes excluded any deletions larger than 500 bp in parietal and entorhinal cortex. It is concluded that there seems to be no specific respiratory chain defect in Alzheimer's disease.