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The mood stabilizer valproic acid (VPA) decreases neural progenitor proliferation and promotes neurogenesis in the adult hippocampus. However, the effects of VPA on progenitor cells in the adult subventricular zone (SVZ) are not as well characterized. Here we report VPA blocks neurosphere formation and inhibits DNA synthesis in cultured NSCs from the SVZ of adult mice. Inhibition of DNA synthesis is associated with the up-regulation of the differentiation transcription factors Egr1 and Neurod1 and down-regulation of transcription factors associated with “stemness”. Co-treatment of VPA with the mood stabilizer lithium antagonizes the anti-proliferative effects of VPA on adult NSCs and abolishes VPA activation of Egr1. Co-treatment of VPA with the MEK1/2 inhibitor PD980589 similarly abolishes Egr1 activation consistent with VPA activation and lithium antagonism of MEK-ERK signaling in adult NSCs. However, Western blot reveals VPA significantly suppresses ERK2 phosphorylation in adult NSCs grown in proliferating culture conditions and that lithium co-treatment does not attenuate this effect. Combined the data indicate VPA inhibition of adult NSC proliferation and activation of Egr1 by VPA, along with the antagonism of these effects by lithium, are the effects of cumulative changes in multiple signaling pathways and are not attributable to a common kinase target.