Calcitriol is a vitamin D3 receptor agonist and active metabolite of vitamin D3 .^1,2 It is formed via a multi-step process, in which vitamin D3 undergoes hydroxylation to 25-hydroxy vitamin D3 in the liver, followed by further hydroxylation at the 1α-position in the kidney. Calcitriol binds to the vitamin D3 receptor (Kd = 16 pM) and induces differentiation of U937 acute monocytic leukemia cells (EC50 = 2,000 pM).³ It induces cell cycle arrest at the S phase and apoptosis in primary adenomatous and hyperplastic human parathyroid cells when used at a concentration of 10 nM.⁴ Calcitriol (0.65 nmol/animal) increases bone calcium mobilization in chicks on a low calcium diet.² It stimulates calcium absorption in the intestines in a chick model of rickets induced by a vitamin D3-deficient diet when administered at a dose of 25 µg/animal.¹ Formulations containing calcitriol have been used in the treatment of hypocalcemia.