Parathyroid gland transplanted rats and hypophysectomized rats were raised from weaning on a diet without vitamin D and low in calcium (0.02%) for 4 weeks. At the enn of this period the animals of both experimental groups, when compared to their respective controls (i.e., sham-operated animals for parathyroid-transplanted ones, and hypophysectomized plus bovine growth hormone-supplemented ones for hypophysectomized rats) were characterized by (a) moderate or absent secondary hyperparathyroidism; (b) near normal bone calcium content; and (c) a maintained responsiveness to the calcemic effect of parathyroid extract (PTE). The PTE action is a bone effect that does not require the presence of the kidneys and is not related to changes in serum calcium and/or phosphorus concentrations. These results indicate that when severe hyperparathyroidism is prevented, the sensitivity of bone to the calcemic action of PTE can be maintained in D-deficient calcium-deprived rats. They also suggest that in these animals the main factor leading to resistance to PTH is the state of severe chronic hyperparathyroidism.