This study evaluated if melatonin would increase bone mass in mice. Four groups of 4-week-old male ddy mice received daily injections of vehicle or 1, 5, or 50 mg/kg of melatonin, respectively, for 4 weeks. Treatment with 5 mg/kg per day or 50 mg/kg per day of melatonin significantly increased bone mineral density (BMD; by 36%, p < 0.005) and bone mass (bone volume per tissue volume [BV/TV] by 49%, p < 0.01, and trabecular thickness [Tb.Th] by 19%, p < 0.05). This treatment significantly reduced bone resorption parameters (i.e., osteoclast surface [Oc.S/bone surface [BS]] by 74%,p < 0.05, and osteoclast number [N.Oc/BS] by 76%,p < 0.005) but did not increase histomorphometric bone formation parameters (i.e., bone formation rate [BFR/ BS], mineral apposition rate [MAR], and osteoid volume [OV/TV]), indicating that melatonin increases bone mass predominantly through suppression of bone resorption. Melatonin (1-500 microM) in vitro caused dose-dependent reduction (p < 0.001 for each) in the number and area of resorption pits formed by osteoclasts derived from bone marrow cells but not those formed by isolated rabbit osteoclasts. Because RANKL increases, while osteoprotegerin (OPG) serves as a soluble decoy receptor for RANKL to inhibit osteoclast formation and activity, the effect of melatonin on the expression of RANKL and OPG in mouse MC3T3-E1 osteoblastic cells was investigated. Melatonin (5-500 microM) increased in a dose-dependent manner and reduced the mRNA level of RANKL and both mRNA and protein levels of OPG in MC3T3-E1 cells (p < 0.001 for each). In summary, these findings indicated for the first time that melatonin at pharmacologic doses in mice causes an inhibition of bone resorption and an increase in bone mass. These skeletal effects probably were caused by the melatonin-mediated down-regulation of the RANKL-mediated osteoclast formation and activation.