It is generally well accepted that the pubertal surge in estrogen is responsible for the rapid bone accretion that occurs during puberty and that this effect is mediated by an estrogen-induced increase in growth hormone (GH)/insulin-like growth factor (IGF) action. To test the cause and effect relationship between estrogen and GH/IGF, we evaluated the consequence of ovariectomy (OVX) in prepubertal mice (C57BL/6J mice at 3 wk of age) on skeletal changes and the GH/IGF axis during puberty. Contrary to our expectations, OVX increased body weight (12-18%), bone mineral content (11%), bone length (4%), bone size (3%), and serum, liver, and bone IGF-I (30 -50%) and decreased total body fat (18%) at 3 wk postsurgery. To determine whether estrogen is the key ovarian factor responsible for these changes, we performed a second experiment in which OVX mice were treated with placebo or estrogen implants. In addition to observing similar results compared with our first experiment, estrogen treatment partially rescued the increased body weight and bone size and completely rescued body fat and IGF-I levels. The increased bone accretion in OVX mice was due to increased bone formation rate (as determined by bone histomorphometry) and increased serum procollagen peptide. In conclusion, contrary to the known estrogen effect as an initiator of GH/IGF surge and thereby pubertal growth spurt, our findings demonstrate that loss of estrogen and/or other hormones during the prepubertal growth period effect leads to an increase in IGF-I production and bone accretion in mice. osteoporosis; estrogen; puberty; bone size; bone mineral density OSTEOPOROSIS IS A MAJOR HEALTH concern in humans, and this disease is characterized by a low bone mineral density (BMD), which leads to increased bone fragility and risk of fractures. BMD is an important determinant of bone strength, and low BMD is the result of increased bone resorption and decreased bone formation. It has been well established that the pubertal growth period is a critical period in time when rapid increase in bone accretion takes place. We and others (19,32,48) have demonstrated in both humans and mice that 40 -50% of bone accretion occurs during puberty. Understanding the mechanisms involved in regulating bone accretion during this period of rapid growth is of considerable importance in the prevention and treatment of osteoporosis.In terms of the potential messenger molecules that contribute to rapid skeletal growth during puberty, it has been suggested that the growth hormone (GH)/insulin-like growth factor (IGF) axis is a key regulator of early bone development (34, 55). Recent studies using transgenic mouse models lacking IGF-I or GH have provided convincing evidence for a key role for GH/IGF axis in the regulation of skeletal growth that occurs during puberty (28). It is well accepted that increased IGF-I during sexual maturation is regulated by increased GH production and/or action as demonstrated by reduced IGF-I and bone density in mice lacking GH (28). In addition, we and ot...