['The phytoectysteroid ecdysterone (Ecdy) was reported to stimulate protein synthesis and enhance physical performance. The aim of this study was to investigate underlying molecular mechanisms particularly the role of ER beta (ERβ).', 'In male rats, Ecdy treatment increased muscle fiber size, serum IGF-1 increased, and corticosteron and 17β-estradiol (E2) decreased. In differentiated C2C12 myoblastoma cells, treatment with Ecdy, dihydrotestosterone, IGF-1 but also E2 results in hypertrophy. Hypertrophy induced by E2 and Ecdy could be antagonized with an antiestrogen but not by an antiandrogen. In HEK293 cells transfected with ER alpha (ERα) or ERβ, Ecdy treatment transactivated a reporter gene. To elucidate the role of ERβ in Ecdy-mediated muscle hypertrophy, C2C12 myotubes were treated with ERα (ALPHA) and ERβ (BETA) selective ligands. Ecdy and BETA treatment but not ALPHA induced hypertrophy. The effect of Ecdy, E2, and BETA could be antagonized by an ERβ-selective antagonist (ANTIBETA). In summary, our results indicate that ERβ is involved in the mediation of the anabolic activity of the Ecdy.'] Is estrogen receptor beta involved in skeletal muscle hypertrophy induced by the phytoecdysteroid ecdysterone?