1. The response to exogenous parathyroid hormone (PTH) was tested in normal subjects and patients with osteomalacia due to vitamin D deficiency; 200 MRC units of bovine PTH were administered intravenously.
2. The rise in plasma adenosine 3′:5′-cyclic monophosphate (cyclic AMP) and the increase in urinary excretion of cyclic AMP were reduced in the patients with vitamin D deficiency. After treatment with vitamin D the responses returned to normal.
3. It is suggested that this reversible resistance is due to the secondary hyperparathyroidism associated with vitamin D deficiency.