Developmental shift in GABA actions from depolarization to hyperpolarization occurs as a result of decreasing the intracellular Cl- concentration regulated by K+-Cl- co-transporter 2 (KCC2). To clarify the time-course of the developmental shift on the Purkinje cells, we examined KCC2-localization in the embryonic mouse cerebellum. The KCC2 was first detected within the Purkinje cells in the Purkinje cell layer of the hemisphere at embryonic day 15 (E15) and the vermis at E17, but the ventricular and intermediate zones were negative. These results suggest that GABA might become inhibitory on the Purkinje cells after their settling in the Purkinje cell layer. © 2006 Elsevier Ireland Ltd and the Japan Neuroscience Society.