There is accumulating evidence suggesting that estrogen receptor (ER)-β may play a role different from ER-α in the regulation of various social behaviors in both sexes. Recently, we found that ER-β knockout (βERKO) female mice showed greatly elevated levels of postpartum aggression toward male intruder mice (Le et al., SFN abstract 2002). This finding suggests that ER-β may be involved in the regulation of postpartum aggression in mice. However, the profile of ER-β gene expression in mouse brain during the postpartum period remains unclear. Thus, we examined the expression of ER-β gene in female mice on postpartum days (PPD) 1, 5, 9, and 25 in four forebrain regions; the medial preoptic area (MPOA), the paraventricular nucleus (PVN), the bed nucleus of the stria terminalis (BNST), and the amygdala (AMY), in which ER-β is known to be abundant. In situ hybridization histochemistry revealed that the ER-β gene expression in the MPOA gradually increased. In the PVN and the BNST, it increased between PPD-1 and 5...