The influence of sodium fluoride (NaF) on calcium metabolism was examined in goldfish (fresh water teleost). At 2 days after administration of NaF (500 ng/g body weight; 5µg/g body weight)(around 10 -5 to 10 -4 M in goldfish), we indicated that plasma calcium levels upregulated in both doses of NaF-treated goldfish. To examine the mechanism of hypercalcemia by NaF treatments, therefore, direct effects of NaF on osteoblasts and osteoclasts in goldfish were investigated by an original assay system using teleost scale which has osteoblasts, osteoclasts and bone matrix. Alkaline phosphatase activity in the scales increased with the treatment of NaF (10 -6 and 10 -5 M) during 6 hours of incubation. Also, tartrate-resistant acid phosphatase activity increased after exposure to NaF (10 -5 M) at the 6 hours of incubation. To investigate the osteoclastic activation, the mRNA expression of osteoclastogenesis related factors were examined. The receptor activator of the nuclear factor-κB ligand (RANKL) which is known as a factor for osteoclastogenesis, increased in the NaF-treated scales after 6 hours of incubation.The ratio of RANKL/osteoprotegerin (osteoclastogenesis inhibitory factor) significantly increased after 6 hours of incubation. Resulting from the increase of RANKL mRNA level, the expression of transcription-regulating factors was significantly increased. Furthermore, the expression of functional genes, cathepsin K and matrix metalloproteinase-9 mRNA, was significantly increased. In our knowledge, this is the first report concerning the effects of NaF on osteoblasts and osteoclasts in teleosts. We concluded that NaF influences calcium metabolism via osteoclastic activation in goldfish.3 Keywords: sodium fluoride; osteoblasts; osteoclasts; scales; calcium metabolism; RANKL;OPG; goldfish Abbreviations: ALP, alkaline phosphatase; CTSK, cathepsin K; MMP-9, matrix metalloproteinase-9; NFATc1, nuclear factor of activated T-cells and cytoplasmic 1; TRAF6, TNF receptor-associated factor 6; TRAP, tartrate-resistant acid phosphatase; OPG, osteoprotegerin; RANK, receptor activator of the nuclear factor-κB ; RANKL, receptor activator of the nuclear factor-κB ligand 4