The multiple cyclins and their catalytic subunit, cdc2-related kinase, play essential roles in the eucaryotic cell cycle. To examine their potential roles in highly differentiated nervous system, we determined their activity and localization in rat brain. p13suc1 associated histone H1 kinase activity was dramatically increased at the onset of brain maturation. The increased kinase activity was coprecipitated with cyclin D1, a type of G1 cyclin, which also increased with brain maturation. Immunohistochemical analysis demonstrated that anti-cyclin D1-like immunoreactivity was exclusively localized to neurons. From these findings, it is suggested that a type of cdc2-related kinase regulated by cyclin D1 might function in neurons independent of cell cycle progression.