Using quantitative autoradiography, both nuclear DNA repair - measured as nuclear unscheduled DNA synthesis (UDS) - and mitochondrial (mt) DNA synthesis were evaluated in situ for several types of cells in the brains of untreated mice of various age. It was found that distinct types of neuronal cells showed a decline of both UDS and mtDNA synthesis with age, whereas - except for glial cells of the cerebral cortex - no glial or endothelial cells showed age-related alterations of UDS. Together with various data reported in the literature, these patterns of a cell type-specific decrease of UDS and mtDNA synthesis with age in the mouse brain lead to an improved understanding of the complex interrelationships between the molecular events associated with the phenomenon of aging as well as to a new idea regarding the cause of the specific distribution pattern of those cells in the human brain that are affected by the formation of paired helical filaments in Alzheimer's disease.