We have elucidated the spatial and temporal localization of Cdk5 and p35nck5a in the developing rat postnatal cerebellum. Both proteins were highly expressed in cell bodies of post mitotic and immature neurons. The localization of Cdk5 in cellular compartment was changed from cell body to the axon in development. On the other hand, p35nck5a was always expressed in the cell body throughout cerebellum development. The Cdk5 kinase activity was correlated with the expression of p35nck5a rather than that of Cdk5. These results indicate that p35nck5a is a physiological activator of Cdk5 in immature neurons and further suggest that Cdk5 has another function in mature neurons.