1. Five patients with chronic hypoparathyroidism were given dihydrotachysterol.
2. Calcium and phosphorus absorption and excretion were measured in three of the patients. An increased absorption of calcium from the intestine was the major source of the increased excretion of calcium after dihydrotachysterol.
3. A change in the renal tubular handling of phosphorus was a major factor in the changes in the serum phosphorus after treatment with dihydrotachysterol.
4. The results suggest that there is a difference between organ responses in any one individual, and that differences in a given organ response between individuals cannot be entirely attributed to the dose of dihydrotachysterol.
5. The implications of these observations for the treatment of hypoparathyroidism are discussed.