1. Protein-binding assays have been used to measure plasma 1,25-dihydroxy-vitamin D [1,25-(OH)2D] as well as 25-hydroxy-vitamin D [25-(OH)D] in rats given 10 mg of phosphorus (P) day−1 kg−1 as ethane-1-hydroxy-1,1-diphosphonate (EHDP).

2. In control animals given a normal laboratory chow plasma 25-(OH)D and 1,25-(OH)2D were about 40 nmol/l and 300 pmol/l respectively.

3. EHDP produced a decrease of plasma 1,25-(OH)2D to below 50 pmol/l in 2 days.

4. Both in control and in EHDP-treated rats plasma 1,25-(OH)2D increased when dietary calcium (Ca) was restricted to 0.1%, or dietary P to 0.2%, indicating that the well-known stimulatory effect of Ca or P deprivation was at least partially effective in EHDP-treated rats.

5. In response to an increase of the oral supply of vitamin D3 to 65 nmol/day the plasma level of 25-(OH)D rose in both control and EHDP groups. Plasma 1,25-(OH)2D was not increased above the normal value in control rats. In EHDP-treated rats, however, plasma 1,25-(OH)2D rose to a level equal to that in controls, suggesting that the effect of EHDP on plasma 1,25-(OH)2D can be overcome at high precursor concentration.

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