Transcription of the CYP24 (25-hydroxyvitamin D₃-24-hydroxylase) gene is known to be induced by 1α,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃). We studied the induction kinetics in detail in human skin-derived fibroblasts. While the basal transcription of this gene was very low, addition of 1,25(OH)₂D₃ increased the mRNA level by 50-fold within 1h. The induction reached as high as 20000-fold after 12h. DNA microarray analysis also indicated that the induction ratio of the CYP24 gene is exceptionally high among 3800 human genes examined. The increase of mRNA was caused by stimulation of the transcription, but not by stabilization of mRNA. 24(R),25-dihydroxyvitamin D₃ (24,25(OH)₂D₃), a compound metabolically related to 1,25(OH)₂D₃, also stimulated the CYP24 gene transcription, though at much higher concentrations. However, this stimulation was significantly augmented by synergistic actions of 24,25(OH)₂D₃ and 1,25(OH)₂D₃, suggesting that 24,25(OH)₂D₃ or its metabolites might be playing some roles in the regulation of CYP24 gene transcription.