Fibroblast growth factor-23 (FGF-23), a novel phosphate-regulating factor, was elevated in hypophosphatemic patients with X-linked hypophosphatemic rickets/osteomalacia and also in patients with chronic kidney disease. These observations suggested the pathophysiological importance of FGF-23 on phosphate homeostasis. However, regulation of FGF-23 production is still unclear. We investigated effects of both dietary phosphorus and 1alpha,25-dihydroxyvitamin D(3) (1alpha,25(OH)(2)D(3)) on circulating FGF-23 in vivo Administration of. 1alpha,25(OH)(2)D(3) dose-dependently increased serum FGF-23 in thyroparathyroidectomized rats without correlating with serum inorganic phosphorus or serum parathyroid hormone. On the other hand, vitamin D receptor null mice had very low serum FGF-23 and did not respond to the 1alpha,25(OH)(2)D(3) administration. These observations suggested 1alpha,25(OH)(2)D(3) directly or indirectly regulates circulating FGF-23. Serum FGF-23 had a strong correlation with serum inorganic phosphorus controlled by dietary phosphorus in 5/6 nephrectomized rats. High phosphate diet elicited a 5-fold increase in serum FGF-23 compared with sham-operated rats, whereas serum FGF-23 did not correlate with serum calcium or serum creatinine in 5/6 nephrectomized rats. Administration of 1alpha,25-dihydroxyvitamin D(3) also elicited a severalfold increase in serum FGF-23 in the uremic rats. Taken together, this shows that both serum phosphorus and 1alpha,25(OH)(2)D(3) regulate circulating FGF-23 independent of each other. Therefore, we proposed there was a feedback loop existing among serum phosphorus, 1alpha,25(OH)(2)D(3), and FGF-23, in which the novel phosphate-regulating bone-kidney axis integrated with the parathyroid hormone-vitamin D(3) axis in regulating phosphate homeostasis.