Nephrectomized vitamin D-defkient rats on a low-calcium diet show the same intestinal calcium transport response to 1,25-dihydroxyvitamin D3 whether parathyroid hormone is present or not. Additionally, vitamin D-deficient rats on a low phosphorus diet give an intestinal calcium transport response equal to sham-operated controls even 4 days after thyroparathyroidectomy. On the other hand, only a partial mobilization of calcium from body stores in response to 1,25-dihydroxyvitamin D3 is noted in thyroparathyroidectomized rats. This response can be restored with exogenous parathyroid extract (10 units every 6 hr). Thus parathyroid hormone has a direct function in mobilization of calcium from body stores in addition to stimulating 1,25-dihydroxyvitamin D3 synthesis. These results strongly support the idea that parathyroid hormone has no direct action on intestinal calcium absorption, but it stimulates this system by stimulating the synthesis of 1,25-dihydroxyvitamin D3(Endocrinology 94: 1022, 1974)