The orphan nuclear estrogen receptor-related receptor α (ERRα) is expressed by osteoblastic cells and plays a functional role in osteoprogenitor proliferation and differentiation. To dissect further the role of ERRα in bone, we investigated the effects of estrogen (E2) on ERRα both in vitro and in vivo. Chronic treatment of fetal rat calvaria cells with E2-stimulated bone nodule formation and up-regulated ERRα mRNA expression at early (10 h and d 8) but not later times in culture, suggesting a link between ERRα and E2 during osteoprogenitor proliferation. ERRα mRNA levels were significantly lower in ovariectomized adult rat bones vs. those of sham-operated rats early (1 d and 1 wk) post surgery, but levels returned to control levels thereafter. ERRα is also expressed in osteoclasts (tartrate-resistant acid phosphatase + multinucleated cells) in vivo and in vitro (RAW 264.7 cells) and ovariectomization lowered the OPG/receptor activator of nuclear factor κB ligand expression ratio. Down-regulation of ERRα expression via antisense treatment of rat calvaria cells not only inhibited osteogenesis but also increased adipocyte colony formation and changed the OPG/receptor activator of nuclear factor κB ligand ratio. These data suggest that ERRα is regulated by estrogen in bone in which it may play a functional role at several levels (osteoblasts, adipocytes, and osteoclasts) in E2 deficiency diseases such as osteoporosis.