Two patients with vitamin D resistant rickets respond to a high phosphorus intake by a positive calcium balance. The response to a calcium infusion suggests a parathyroid effect on phosphaturia but the size and histology of the parathyroid glands in one patient was normal. A circadian phosphorus excretion rhythm was present.Studies of the effect of glucose on the renal handling of phosphorus in these patients suggest an inhibitory effect which is lacking in normals. It is postulated that one of two metabolic pathways for the renal tubular reabsorption of phosphorus is absent in vitamin D resistant rickets.