Total DNA and RNA contents and the activity of butyrylcholinesterase, a nonspecific cholinesterase, were determined in the cerebral hemispheres, cerebellum, diencephalon-midbrain, and optic lobes in chicks from embryonic age and posthatching up to 36 mo. In both the cerebellum and the cerebral hemispheres DNA content decreased during embryonic age, reflecting a decrease in cell density due to growth in cell size, increased from 6 weeks up to 20 mo. Posthatching, reflecting proliferation of glial cells, and decreased after 20 mo., indicating loss of neurons during aging. RNA was at high levels in all four CNS structures studied during embryonic development, reflecting the involvement of RNA in cellular growth. BuChE activity progressively increased in all four CNS structures studied from embryonic age up to 3 mo. After hatching. In the cerebellum and cerebral hemispheres BuChE dropped to very low levels at 12 mo. And progressively increased thereafter. BuChE is predominantly localized in glial cells. Thus, the increase in BuChE activity during aging is interpreted to reflect the increased proliferation of gial cells with aging.