THE aged central nervous system (CNS) is characterized by a loss of neurons. Apoptosis has been reported to be responsilbe for neuronal death during development and may also be involved in some age-related neurodegenerative diseases of the CNS. No evidence is currently is available as to whether apoptosis is also responsible for the loss of neurons associated with physiological aging. In this study, we have found fragmented DNA, a characteristic trait of cells undergoing apoptosis, in the hippocampus and, to a minor extent, in the frontal cortex and basal forebrain of aged (24-month-old) Fisher-344 rats, but not in the cerebellum. These data represent important evidence to support the idea that apoptosis may be associated with the neuronal death observed during normal aging.