Proliferative oligodendrocyte progenitor cells (OPs) express large, delayed outward-rectifying K ؉ currents (IK), whereas nondividing immature and mature oligodendrocytes display much smaller I K. Here, we show that up-regulation of IK occurs in G1 phase of the cell cycle in purified cultured OPs and is the result of an RNA synthesis-dependent, selective increase of the K ؉ channel subunit proteins Kv1.3 and Kv1.5. In oligodendrocyte cells acutely isolated from developing rat brain, a decrease of cyclin D expression is observed as these cells mature along their lineage. This is accompanied by a decrease in Kv1.3 and Kv1.5 subunit expression, suggesting a role for these subunits in the proliferative potential of OPs in situ. IK expressed in OPs in subventricular zone and developing white matter in acutely isolated slice preparations were selectively blocked by antagonists of Kv1.3, illustrating the functional presence of this subunit in situ. Interestingly, Kv1.3 block inhibited S-phase entry of both purified OPs in culture and in tissue slice cultures. Thus, we employ both in vitro and in situ experimental approaches to show that (i) RNA-dependent synthesis of Kv1.3 and Kv1.5 subunit proteins occurs in G 1 phase of the OP cell cycle and is responsible for the observed increase in I K, and (ii) currents through Kv1.3-containing channels play a crucial role in G 1͞S transition of proliferating OPs. O ligodendrocytes, a major class of macroglia, are responsible for myelination in the central nervous system and primarily originate from highly proliferative oligodendrocyte precursor cells (OPs) in a spatially restricted central nervous system area termed the subventricular zone (SVZ; refs. 1 and 2). Under the influence of extrinsic trophic signals (3, 4), OPs migrate, proliferate, and the majority differentiate (by means of a number of intermediate cell stages) into myelinating oligodendrocytes.A correlation exists between expression of the delayed, outward-rectifying voltage-gated K ϩ currents (I K ) and the proliferative potential of oligodendrocyte lineage cells (5-9). Proliferating OPs possess large I K , whereas postmitotic oligodendrocytes do not express such currents (5-9). However, few studies have attempted to identify the cellular mechanisms responsible for these K ϩ channel changes in OPs. Furthermore, although the functional properties of these channels indicate that they are composed of subunits of the Kv1 subfamily (9, 10), the molecular identity of the K ϩ channel subunits involved in the developmental alterations of K ϩ channel expression in OPs has not been extensively analyzed.We have previously used a cell culture model system that allows OPs to be synchronized in a nonproliferative state (G 0 ) and be induced to enter the cell cycle upon mitogen treatment (11,12). By using this approach, we have previously shown that mitogen-induced entry of OPs into the cell cycle is accompanied by a marked increase in I K . § In the present study, we take further advantage of this culture system to: (i) analyze ...