Some epigenetic mechanisms, including DNA methylation and histone deacetylation, act as transcriptional repression signals. In this study, we examined whether DNA methylation dependent transcriptional control regulates glial cell growth. Primary cultured mouse cortical glial cells were treated with the DNA methylation inhibitor 5-aza-deoxycytidine (5adC) or the histone deacetylase inhibitor sodium valproate (VPA), which inhibits DNA-methylation-dependent transcriptional repression. 5adC significantly reduced methylated C level determined by reversed-phase high-performance liquid chromatography (HPLC), while VPA did not. Treatments with these inhibitors significantly reduced cell number determined by MTT assay after 48 h. Both 5adC and VPA showed little cellular toxicity observed by live and dead cell staining. In contrast, both 5adC and VPA induced an abnormality in the cell cycle. Cells treated with the inhibitors represented a significantly higher ratio in the G2+M-phase and 5adC-treated cells showed a significantly lower ratio in the S-phase. Regarding the in vivo effect, prenatal treatment with VPA, which is an autistic model in rodents, significantly reduced the brain/body weight ratio in early postnatal days. Our data indicate that DNA-methylation-and histone-deacetylation-dependent transcriptional control is crucial for the regulation of glial cell growth. Our data suggest that abnormalities of epigenetic transcriptional regulatory mechanisms in glial cells cause an abnormal brain size, which may in turn cause mental diseases.