Acquisition of social dominance is important for social species including mice, for preferential access to foods and mates. Male mice establish social rank through agonistic behaviors, which are regulated by gonadal steroid hormone, testosterone, as its original form and aromatized form. It is well known that estrogen receptors (ERs), particularly ER alpha (ER alpha), mediate effects of aromatized testosterone, i.e., 17 beta-estradiol, but precise role played by ER beta (ER beta) is still unclear. In the present study, we investigated effects of EF beta gene disruption on social rank establishment in male mice. Adult male ER beta knockout (beta ERKO) mice and their wild type (WT) littermates were paired based on genotypeand weight-matched manner and tested against each other repeatedly during 7 days experimental period. They underwent 4 trials of social interaction test in neutral cage (homogeneous set test) every other day. Along repeated trials, WT but not beta ERKO pairs showed a gradual increase of agonistic behaviors including aggression and tail rattling, and a gradual decrease of latency to social rank determination in tube test conducted after each trial of the social inter