THE levels and cellular localization of the mRNA encoding the inwardly rectifying potassium ion channel Kir4.1 were investigated in the embryonic rat brain by Northern blots andin situhybridization. This transcript was absent at embryonic day 13 (E13), whereas it was clearly present in E14–15 preparations, principally in the neuroepithelium of the cerebral cortex, thalamus, and hypothalamus. At later embryonic stages (E17–20), Kir4.1 mRNA levels increased and expanded to the mantle zone, such as the cortical plate, hippocampus, thalamus, and hypothalamus. The early appearance of Kir4.1 mRNA in various brain regions suggests an involvement of the channel in cell proliferation, migration and differentiation in the rat CNS.