Isolated renal tubules from vitamin D-deficient chicks catalyse the in vitro conversion of 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol. This conversion is stimulated by 5 × 10^-10 M bovine parathyroid hormone, or by 10^-6 M cyclic AMP. It is inhibited by 10^-9 M porcine calcitonin. It is concluded that these hormonal controls of the synthesis of the renal hormone 1,25-dihydroxycholecalciferol are of particular physiological significance in coordinating the activities of the various organs involved in extracellular calcium homeostasis.