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1. The role of parathyroid hormone (PTH) and 1, 25-dihydroxy-cholecalciferol (1,25-(OH)2D3) in modulation of intestinal Ca absorption was studied in rats, using disodium ethane-1-hydroxy-1,1-diphosphonate (EHDP), which is known to reduce 1,25-(OH)2D3 formation. 2. EHDP decreased intestinal Ca absorption. This effect could be abolished by small amounts of 1,25-(OH)2D3, whereas even large doses of PTH were ineffective. EHDP also decreased Ca absorption in thyroparathyroidectomized (TPTX) rats. Therefore the effect of EHDP on 1,25-(OH)2D3 production is unlikely to be mediated through PTH. 3. The correction by PTH of the decreased Ca absorption in TPTX rats was inhibited by EHDP. Since EHDP inhibits formation of 1,25-(OH)2D3 the effect of PTH on Ca absorption is likely to be mediated through this vitamin D3 metabolite. 4. In normal rats both a low Ca and a low P diet stimulated Ca absorption. In EHDP-treated intact rats low Ca still stimulated Ca absorption, whereas the effect of low P abolished. This indicates that low Ca and low P diets affect Ca absorption through different mechanisms. 5. Intestinal adaptation to a low Ca diet was still observed in EHDP-treated TPTX rats. Thus, in the rat, intestinal adaptation to low Ca diet can occur without PTH.