The regional and tissue-specific expression of the Ca2+/calmodulin-dependent protein kinase, CaM kinase-Gr, were examined. The Mr 65,000 alpha-polypeptide of CaM kinase-Gr is expressed ubiquitously in different anatomical regions of rat brain, whereas an additional Mr 67,000 beta-polypeptide is observed solely in the cerebellum. The alpha-polypeptide appears in the neonatal rat forebrain and cerebellum, whereas the beta-polypeptide appears by the second postnatal week and may reflect cerebellar granule cell differentiation. Most peripheral tissues do not express either CaM kinase-Gr polypeptide. However, rat thymus and thymocytes derived therefrom express CaM kinase-Gr at levels comparable to those of the central nervous system. The identity of the enzyme in rat thymus was corroborated by immunoblot assays, Northern blots, and direct enzyme purification. Rat spleen and testis also produce CaM kinase-Gr, but at lower levels than either thymus or brain. These observations demonstrate selective regional and developmental expression of CaM kinase-Gr polypeptide in brain, and suggest that it may participate in Ca2+ signalling in cells derived both from the immune system as well as the central nervous system.