1. One-year-old male rats were injected intravenously with 200 pmol of 25-hydroxy[26(27)-methyl-3H]cholecalciferol per 100 g body weight and the presence of this metabolite of vitamin D, as well as other metabolites, produced during the following 8 h was examined in serum, urine and bile. 2. The chromatography data indicated an excretion of 25-hydroxycholecalciferol both in bile and urine and, in urine, also of 1,25-dihydroxycholecalciferol. In bile, fractions of labelled substances were also obtained which, according to their elution positions, might represent cholecalciferol and conjugated metabolites. 3. The excretion of active metabolites of vitamin D in normal urine might be elevated in chronic renal failure and, in conjunction with a reduced synthesis, contribute to the occurrence of renal osteodystrophy.