Kv4.3, an A-type K+ channel, is the only channel molecule showing anterior-posterior (A-P) compartmentalization in the granular layer of mammalian cerebellum known so far. Kv4.3 mRNA has been detected from the posterior but not anterior granular layer in adult rat cerebellum. To characterize this A-P compartmentalization further, we examined Kv4.3 protein expression in rat cerebellum by immunohistochemistry at the embryonic, early postnatal and adult stages. Specificity of the Kv4.3 antibody was confirmed by both Western blot and immunoprecipitation analysis. In adulthood, Kv4.3 was detected from the somatodendritic domain of posterior granule cells, with a restriction boundary in the vermal lobule VI extending laterally to the hemispheric crus 1 ansiform lobules. At the early postnatal stage, this A-P pattern first appeared on postnatal day 8, when significant numbers of granule cells had migrated into the posterior granular layer and started to express Kv4.3. Similar Kv4.3 expression in the somatodendritic domain of post-migratory neurons in the cerebellum was also observed in basket cells, stellate cells, a subset of GABAergic deep neurons, Lugaro cells and, probably, deep Lugaro cells. However, none of them showed A-P compartmentalization. Strikingly, we found Kv4.3 in several clusters of migrating Purkinje cells with mediolateral compartmentalization. These Purkinje cells no longer expressed Kv4.3 after completing the migration. By contrasting the expression in migrating and post-migratory neurons, our results suggest that Kv4.3 may play an important role in the development of cerebellum, as well as in the mature cerebellum.