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.