The numbers of neurons and glial cells in the cerebral cortex of the mouse have been estimated during its whole life-span (5 to 720 days), taking into account both the cellular densities of several areas and the cortical volumes. The results clearly demonstrate that there is a massive neuronal loss in the cerebral cortex during early postnatal development, greater in layers II-IV than in layers V-VI. In contrast, aging is characterized by a discrete neuronal loss in the cerebral cortex, purely restricted to layers II-IV. The number of glial cells increases continuously from 5 to 720 days. We emphasize here the need to obtain volumetric measure together with cellular densities in order to get interpretable quantitative data on cellular death and proliferation.