X-linked hypophosphatemic (Hyp) mice exhibit hypophosphatemia, impaired renal phosphate reabsorption, defective skeletal mineralization, and disordered regulation of vitamin D metabolism: In Hyp mice, restriction of dietary phosphorus induces a decrease in serum concentration of 1,25(OH)₂D and renal activity of 25-hydroxyvitamin D-1-hydroxylase (1-hydroxylase), and induces an increase in renal activity of 25-hydroxyvitamin D-24-hydroxylase (24-hydroxylase). In contrast, in wild-type mice, phosphorus restriction stimulates renal 1-hydroxylase gene expression and suppresses that of 24-hydroxylase. To determine the molecular basis for the disordered regulation of vitamin D metabolism in Hyp mice, we determined renal mitochondrial 1-hydroxylase activity and the renal abundance of P450c1 and P450c24 mRNA in wild-type and Hyp mice fed either control, low, or high phosphorus diets for 5 days. In wild-type mice, phosphorus restriction increased 1-hydroxylase activity and P450c1 mRNA expression by 6-fold and 3-fold, respectively, whereas in the Hyp strain the same diet induced changes of similar magnitude but opposite in direction. Phosphorus supplementation was without effect in wild-type mice, whereas in Hyp mice the same diet induced 3-fold and 2-fold increases, respectively, in enzyme activity and P450c1 mRNA abundance. In wild-type mice, both renal 1-hydroxylase activity and P450c1 mRNA abundance varied inversely and significantly with serum phosphorus concentrations, whereas in Hyp mice the relationship between both renal parameters and serum phosphorus concentration was direct. In Hyp mice, phosphorus restriction induced a significant increase in renal P450c24 mRNA abundance, in contrast to the lack of effect observed in wild-type mice. The present findings demonstrate that regulation of both the P450c1 and P45024 genes by phosphorus is disordered in Hyp mice at the level of renal 1-hydroxylase activity and renal P450c1 and P450c24 mRNA expression.