P revious studies have shown that chronically thyroparathyroidectomized (TPTX) rats, fed a diet with restricted calcium but adequate phosphorus and vitamin D content, have higher levels of intestinal calcium absorption than controls. The results of recent acute experiments have suggested that parathyroid hormone (PTH) may be essential for regulating the renal conversion of 25-hydroxyvitamin D₃ (25-OH-D₃) to 1,25-dihydroxyvitamin D₃ [1,25-(OH)₂-D₃] in response to dietary calcium deprivation. Since 1,25-(OH)₂-D₃ is the form of the vitamin thought to be active in the intestine, increases in calcium transport mediated by this metabolite would not be expected to occur in the absence of the parathyroid glands if the preceding model is correct. The present study was undertaken to examine the chronic effects of both dietary calcium restriction and the absence of PTH on the metabolism of [³H]25-OH-D₃ and duodenal calcium-active transport in rats given thyroid replacement. These relatively long term studies confirm earlier observations which indicated that the adaptation of calcium absorption to a low calcium intake occurs in both sham-operated and TPTX animals.The present studies also demonstrated that despite reduced levels of 1,25-(OH)₂-D₃ in the plasma of chronically TPTX animals fed a low calcium diet, the accumulation of this metabolite in at least one target tissue, intestinal mucosa, is identical in both the sham-operated and TPTX groups. A reduced, but continued level of 1,25-(OH)₂-D₃ production, together with its selective accumulation by intestinal mucosa, probably explains the calcium adaptation which is observed inspite of the chronic absence of the parathyroid glands.