The gene bak regulates apoptosis. To explore its role in the human central nervous system, we examined the distribution of its protein product, Bak, in the brains at various ages, by immunochemical and immunohistochemical means. Western blotting revealed that Bak expression in the cerebrum and cerebellum is high in the brains of the fetuses and elderly subjects, but low in those of young adults. Immunostaining of the cerebellum localized Bak immunoreactivity to Purkinje cells, which was strong in the fetal period and senescence, but undetectable from infancy to adolescence. These results suggest that bak regulates neuronal death associated with the development and aging of the central nervous system.