Extracellular signal-regulated kinase 2 (ERK2) is involved in a variety of cell fate decisions during development, but its exact role in this process remains to be determined. To specifically focus on the role of ERK2 in the brain, and to avoid early lethalities, we used a conditional gene-targeting approach to preferentially inactivate Erk2 in the embryonic mouse brain. The resulting mutant mice were viable and were relatively normal in overall appearance. However, the loss of Erk2 resulted in a diminished proliferation of neural stem cells in the embryonic ventricular zone (VZ), although the survival and differentiation of these cells was unaffected. The multipotent neural progenitor cells (NPCs) isolated from ERK2-deficient brains also showed impaired proliferation, reduced self-renewal ability, and increased apoptosis. By neurosphere differentiation analysis we further observed that lineage-restricted glial progenitors were increased in ERK2-deficient mice. The decline in the self-renewal ability and multipotency of NPCs resulting from the loss of ERK2 was found to be caused at least in part by upregulation of the JAK-STAT signaling pathway and reduced G1/S cell cycle progression. Furthermore, by global expression analysis we found that neural stem cell markers, including Tenascin C NR2E1 (Tlx), and Lgals1 (Galectin-1), were significantly downregulated, whereas several glial lineage markers were upregulated in neurospheres derived from ERK2-deficient mice. Our results thus suggest that ERK2 is required both for the proliferation of neural stem cells in the VZ during embryonic development and in the maintenance of NPC multipotency by suppressing the commitment of these cells to a glial lineage.