The monoamine serotonin (5-HT), a well known neurotransmitter, is also important in peripheral tissues. Several studies have suggested that 5-HT is involved in bone metabolism. Starting from our original observation of increased 5-HT 2B receptor (5-HT 2B R) expression during in-vitro osteoblast differentiation, we investigated a putative bone phenotype in vivo in 5-HT 2B R knockout mice. Interestingly, 5-HT 2B R mutant female mice displayed reduced bone density that was significant from age 4 months, and had intensified by 12 and 18 months. This histomorphometrically-confirmed osteopenia seems to be due to reduced bone formation since (1) the CFU-F ALP+ capacity of bone marrow precursors was markedly reduced in the 5-HT 2B receptor mutant mice from 4 to 12 months of age,(2) ex-vivo primary osteoblasts from mutant mice exhibited reduced proliferation and delayed differentiation, and (3) calcium incorporation was markedly reduced in osteoblasts after 5-HT 2B R depletion (produced genetically or by pharmacological inactivation). These findings support the hypothesis that the 5-HT 2B R receptor facilitates osteoblast recruitment and proliferation, and that its absence leads to osteopenia that worsens with age. We show here, for the first time, that the 5-HT 2B receptor is a physiological mediator of 5-HT in bone formation and, potentially, in the onset of osteoporosis in aging women.