The vitamin D metabolites, 25-hydroxyvitamin D (25OHD) and 1,25-dihydroxyvitamin D [1,25-(OH)2D], were measured in samples of plasma from normal and X-linked hypophosphatemic (Hyp) mice, which are a model for this common form a human vitamin D-resistant rickets. Each sample was the pooled plasma from the exsanguination of 10–15 mice. On stock diets plasma 1,25-(OH)2D was the same in Hyp mice as in normal mice [normal vs. Hyp, 70 ± 4 vs. 66 ± 4 pM; (mean ± SE) n = 8; P = NS]. However, plasma 25OHD was lower in Hyp mice (59 ± 4 vs. 37 ± 4 nM; n = 6; P < 0.01). Since hypophosphatemia usually elevates plasma 1,25-(OH)2D in rats, we suspected that Hyp mice were unresponsive to hypophosphatemia. To test this, mice were challenged with a low phosphorus (P) diet for 6 days. A low P diet lowered plasma P and raised plasma calcium levels in both normal and Hyp mice. In both genotypes, the low P diet also increased magnesium levels in urine and transiently in plasma. The low P diet raised plasma 1,25-(OH)2D in normal mice, but lowered it in Hyp mice to nondetectable levels. There was no significant effect of the low P diet on plasma levels of 25OHD. We conclude that Hyp mice have a defective control system for plasma levels of 1,25-(OH)2D that does not respond to a low P stimulus with elevated plasma levels of the hormone.