A new radioreceptor assay was used to quantify changes in serum concentration of 1alpha,25-dihydroxyvitamin D3 in rats with low calcium or low phosphate diets. Low availability of either ion elicits a fivefold increase in the circulating concentration of 1alpha,25-dihydroxyvitamin D3. The enhancement of 1alpha,25-dihydroxyvitamin D3 concentration in response to calcium deficiency is dependent on the presence of the parathyroid or thyroid glands (or both), suggesting that this effect is mediated by parathyroid hormone. In contrast, the response of phosphate deficiency is independent of these glands and may result from an action of low serum phosphate concentration or some factor associated with phosphate depletion on the renal synthesis of the 1alpha,25-dihydroxyvitamin D3 hormone.