Normal plasma calcium (Ca2+) and phosphate (Pi) concentrations are controlled by the classic hormones, parathyroid hormone and 1,25(OH)2D3. Recently, fibroblast growth factor-23 (FGF-23) has been identified as a hypophosphatemic hormone which is synthesized by osteoblasts and osteocytes in bone tissues (1). Its main function is to increase urinary Pi excretion (2). However, whether FGF-23 regulates Ca2+ metabolism is not known. Since FGF receptors and co-receptor (α klotho) are also present in intestinal epithelial cells (3, 4), FGF-23 might effect intestinal function. Furthermore, because FGF-23 can antagonize 1,25(OH)2D3 action, it might compromise 1,25(OH)2D3-dependent Ca2+ absorption. Therefore, the objective of this study was to investigate the effects of FGF-23 on Ca2+ transport in the small intestine of male ICR (Imprinting Control Region) mice and the signaling pathways involved in vitro with Ussing chambers. Mice were pretreated with vehicle or 1 µg/kg 1,25(OH)2D3 (s.c.) for 3 days to elevate baseline Ca2+ absorption followed by i.v. FGF-23 injection. Mice were anesthetized (i.p.) with 70 mg/kg sodium pentobarbitone and euthanized (i.c.) by overdose with sodium pentobarbitone. Duodenal tissue was removed, mounted in Ussing chambers and directly exposed to FGF-23 and/or various inhibitors in the chambers. Net flux of Ca2+ and electrical parameters were measured throughout the 60-min experiment by 45Ca kinetics and voltage clamp techniques, respectively. This study has been approved by the Institutional Animal Care and Use Committee of the Faculty of Science, Mahidol University. Consistent with previous reports (5), pretreatment with 1,25(OH)2D3 significantly enhanced total active duodenal Ca2+ transport. Both injection of mice with FGF-23 and direct administration of FGF-23 into the Ussing chamber significantly decreased the 1,25(OH)2D3-enhanced duodenal Ca2+ absorption, indicating that FGF-23 might act directly on the intestinal epithelium. Inhibitors of FGF receptors (PD-173074), and the kinases; MAPK/ERK, p38 MAPK, and PKC, but not inhibitors of JNK, Src, JAK2, PI3K, or Akt significantly abolished the effect of FGF-23 on 1,25(OH)2D3-induced intestinal Ca2+ absorption. This study shows for the first time that, besides being a hypophosphatemic hormone, FGF-23 might also act as a hypocalcemic hormone through its inhibitory effect on intestinal Ca2+ absorption, via the intracellular signaling pathway involving MAPK/ERK, p38 MAPK and PKC.