When the stimuli by nerve impulses, neurotransmitters, hormones, peptides and growth factors are administered to the neurons, one of the responses of the nerve cells is the enhancement of Ca2+ influx and/or the release of Ca2+ from the intracellular storage site. Ca2+ may be related to several types of neuronal functions such as biosynthesis of neurotransmitters, stimulussecretion coupling of neurotransmitters and hormones, microtubule assembly-disassembly cycle and many metabolic reactions. Although the precise molecular mechanism mediating the actions of Ca2+ in the brain remains to be elucidated, accumulating evidence suggests that the actions of Ca2+ are mediated through Ca2+-binding proteins. The role of troponin C, a Ca2+-binding protein, was extensively studied in the skeletal muscle first. Subsequently calmodulin, a ubiquitous Ca2+-binding protein, was found to be widely distributed in many tissues and to be in involved in a variety of Ca2+-mediated cellular processes. In an attempt to elucidate Ca2+ actions in the central nervous system, we have been studying Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) and calcineurin (Ca2+/ calmodulin-dependent protein phosphatase). These enzymes have many common substrates and, therefore, may be involved in the neuronal functions via phosphorylation and dephosphorylation of specific proteins.