Nephrectomy prevents completely the bone calcium mobilization response to 25-hydroxycholecalciferol. In contrast it does not prevent this response to 1,25-dihydroxycholecalciferol. Because it is known that the kidney is the site of 1,25-dihydroxycholecalciferol formation, these results provide evidence that 1,25-dihydroxycholecalciferol or a further metabolite thereof and not 25-hydroxycholecalciferol is the metabolically active form of vitamin D₃ responsible for bone calcium mobilization.