The duodenal and ileal absorption of 32P was studied by an in vivo method in control chicks and in chicks fed for 2 weeks a diet deficient in calcium or phosphorus. The results showed a highly significant increase in duodenal and ileal phosphate absorption in chicks fed a low calcium diet and a smaller, though also significant increase in duodenal but not ileal phosphate absorption in chicks fed a low phosphorus diet. The adaptation phenomenon concerning intestinal phosphorus absorption is presumably due in part to a compensatory increase in 1,25-dihydroxy-vitamin D3 in both deficient diets. However, much higher renal l-hydroxylase activity was found in chicks fed the low calcium diet than in chicks fed the low phosphorus diet. The results indicate that duodenal and ileal absorption of phosphate is a vitamin D mediated process, but the adaptation to a low phosphorus diet tends to be less efficient than that to a low calcium diet, especially in the ileum.