Age-related changes in resting levels of the free intracellular calcium concentration ([Ca2+]i) as well as alterations of the rise in [Ca2+]i following depolarization have been investigated in acutely isolated brain cells of various regions of the rat brain. Characterization of the Ca2+ responses following KCl depolarization in the hippocampus, cortex, striatum, and cerebellum of young rats revealed significant regional differences in the basal [Ca2+]i level as well as in the KCl-induced rise in [Ca2+]i. However, there was no correlation between both parameters. Resting [Ca2+]i as well as Ca2+ responses after depolarization were lower in the hippocampus and cortex of the aged animals, but not in the striatum or cerebellum. It is concluded that the Ca2+ homeostasis in the first two regions is specially susceptible to the aging process, resulting in a downregulation of [Ca2+]i, probably as a consequence of an enhanced sensitivity of mechanisms regulating transmembraneous Ca2+ fluxes. The cellular Ca2+ homeostasis was altered in a comparable way in rat spleenocytes. The rise in [Ca2+]i in the aged animals following stimulation of lymphocytes with the mitogen phytohemagglutinin (PHA) was significantly reduced in the plateau phase, which is maintained by Ca2+ influx mechanisms. The data indicate that age-related disturbances of the cellular Ca2+ homeostasis may be present in different cell types and seem to affect mainly transmembraneous Ca2+ flux much more than intracellular Ca2+ release.