The biologically active form of vitamin D3, 1,25-dihydroxycholecalciferol is produced by the kidney. The biosynthesis of 1,25-dihydroxycholecalciferol from 25-hydroxycholecalciferol is apparently controlled and important factors in this control are dietary calcium and phosphorus, and parathyroid hormone secretion. The direct effects of vitamin D and its metabolites on renal function are uncertain. Patients with chronic renal failure have defective synthesis of 1,25-dihydroxycholecalciferol. Uremic patients treated with small doses of 1,25-dihydroxycholecalciferol or 1 α-hydroxycholecalciferol show increased intestinal calcium absorption and bone healing.