Two P loop domain potassium (K2P or KCNK) channels produce transmitter-modulated K+ currents that could influence brain development. We mapped by in situ hybridization the expression of the K2P gene family in the developing mouse brain. All the K2P genes had different expression patterns, and it is likely that many neuronal types change their K2P channel subunit composition during development. Fitting with a possible role in the control of cell division, three K2P genes (tandem of P domains in a weak inwardly-rectifying K+ channel-related K+ channel (TREK) -1, TREK-2 and weak inwardly-rectifying K+ channel-related acid-sensitive K+ channel (TASK) -2) had high expression in the embryonic subventricular and ventricular zones, and the tandem of P domains in a weak inwardly-rectifying K+ channel (TWIK) -1, TREK-1, TREK-2 and TASK-3 genes were significantly expressed in the external cerebellar granule cell layer. There were also some clear changes in developmental expression of the K2P genes: for example, TREK-1 goes from high to low expression in post-migratory cerebellar granule cells; TREK-2 has one of the highest expressions in the embryonic and early postnatal brain of any K2P gene, but transcript levels fall strongly in the postnatal periods, except for cerebellar granule cells. TASK-1 and tandem pore domain halothane-inhibited K+ channel (THIK) -2 genes both turn on specifically in post-migratory cerebellar granule cells, whereas the TASK-3 gene, for example, is strongly expressed in pre-migratory cells as well as post-migratory cells. On the other hand, young postnatal dentate granule cells express TWIK-1, TREK-1 and TREK-2 before P7, but TASK-3 expression only begins to become clear in these cells in the second postnatal week. THIK-2 mRNA was up-regulated with TASK-1 and TASK-3 transcripts in cerebella of GABAA receptor alpha6 subunit knockout mice, possibly implying a functional association of THIK-2, TASK-1 and TASK-3.