Reflecting the prime role of 1α,25(OH)₂D₃ in calcium homeostasis, the activity of 25-hydroxyvitamin D₃ 1α-hydroxylase, a key enzyme for 1α,25(OH)₂D₃ biosynthesis, is tightly regulated by 1α,25(OH)₂D₃, PTH and calcitonin. Its significant activity is found in kidney, though the enzymatic activity is also reported in extra-renal tissues. In the present study, we found that the 1α-hydroxylase gene abundantly expresses in kidney, and at low levels in other tissues and in some cell lines. Positive and negative regulations of 1α-hydroxylase gene by PTH, calcitonin, or 1α,25(OH)₂D₃ were observed at transcriptional levels in kidneys of animals and in a mouse proximal tubule cell line. Moreover, the protein kinase A inhibitor abrogated the PTH-mediated positive regulation. In mice lacking the vitamin D receptor, the 1α-hydroxylase gene expression was overinduced, and the inducible effect of either PTH or calcitonin, but not the repression by 1α,25(OH)₂D₃, was evident. Thus, vitamin D receptor is essential for the negative regulation by 1α,25(OH)₂D₃. Moreover, we demonstrate that renal 1α-hydroxylase gene expression in chronic renal failure model rats was decreased and the positive effect by PTH and calcitonin was diminished. The present study demonstrates that PTH and calcitonin positively regulate renal 1α-hydroxylase gene expression via PKA-dependent and independent pathway, respectively, and that 1α,25(OH)₂D₃ negatively regulates it mediated by vitamin D receptor. Furthermore, in a moderate state of chronic renal failure, renal cells expressing the 1α-hydroxylase gene appear to have diminished potential in response to PTH and calcitonin.