In isolation-reared mice, social encounter stimulation induces locomotor hyperactivity and activation of the dorsal raphe nucleus (DRN), suggesting that dysregulation of dorsal raphe function may be involved in abnormal behaviors. In this study, we examined the involvement of dorsal raphe GABAergic dysregulation in the abnormal behaviors of isolation-reared mice. We also studied an epigenetic mechanism underlying abnormalities of the dorsal raphe GABAergic system. Both mRNA and protein levels of GABA B1a , a GABA B receptor subunit, were increased in the DRN of isolation-reared mice, compared with these levels in group-reared mice. In contrast, mRNA levels for other GABAergic system-related genes (GABA A receptor α1, β2 and γ2 subunits, GABA B receptor 1b and 2 subunits, and glutamate decarboxylase 67 and 65) were unchanged. Intra-DRN microinjection of 0.06 nmol baclofen (a GABA B receptor agonist) exacerbated encounter-induced hyperactivity and aggressive behavior, while microinjection of 0.3 nmol phaclofen (a GABA B receptor antagonist) attenuated encounter-induced hyperactivity and aggressive behavior in isolation-reared mice. Furthermore, microinjection of 0.06 nmol baclofen elicited encounter-induced hyperactivity in group-reared mice. Neither baclofen nor phaclofen affected immobility time in the forced swim test and hyperactivity in a novel environment of isolation reared mice. Bisulfite sequence analyses revealed that the DNA methylation level of the CpG island around the transcription start site (TSS) of GABA B1a was decreased in the DRN of isolation-reared mice. Chromatin immunoprecipitation analysis showed that histone H3 was hyperacetylated around the TSS of GABA B1a in the DRN of isolation-reared mice. These findings indicate that an increase in dorsal raphe GABA B1a expression via epigenetic regulation is associated with abnormal responses to social stimulation such as encounter-induced hyperactivity and aggressive behavior in isolation-reared mice.