The principal nucleus of the bed nucleus of the stria teminalis (BNSTp) is larger and contains more cells in male mice than in females. These sex differences arise from a higher rate of cell death during early postnatal life in females. Perinatal differences in testosterone appear to create this difference because neonatal testosterone treatment reduces cell death in females to the level seen in males. There is a delay of about six days between testosterone exposure and the peak of cell death, indicating that cells somehow ‘remember’ whether or not they have been exposed to high levels of testosterone. This suggests that epigenetic mechanisms control cell fate. We examined whether histone acetylation, which is typically associated with activation of gene expression, plays a role in the sexual differentiation of the BNSTp. We manipulated the balance between histone acetylation and deacetylation by treating animals with the histone deacetylase inhibitor valproic acid (VPA) at the critical time for sexual differentiation. Males, females, and females treated neonatally with testosterone were treated with 50mg/kg VPA or saline on postnatal days 1 and 2. Animals were sacrificed on postnatal day 21 and volume and cell number of the BNSTp was determined. VPA treatment did not influence volume or cell number in control females but significantly reduced both of these parameters in males and testosterone-treated females, thereby eliminating the sex difference. No volume changes were noted in the suprachiasmatic nucleus, suggesting that the VPA effect was specific to the BNSTp. These findings suggest that a disruption in histone deacetylation blocks the masculinizing actions of testosterone in the BNSTp.