[1,2-3H2]Cholecalciferol has been synthesized with a specific radioactivity of 508mCi/mmol by using tristriphenylphosphinerhodium chloride, the homogeneous hydrogen catalyst. With doses of 125ng (5i.u.) of [4-14C,1-3H2]cholecalciferol the tissue distribution in rachitic rats of cholecalciferol and its metabolites (25-hydroxycholecalciferol and peak P material) was similar to that found in chicken with 500ng doses of the double-labelled vitamin. The only exceptions were rat kidney, with a very high concentration of vitamin D, and rat blood, with a higher proportion of peak P material, containing a substance formed from vitamin D with the loss of hydrogen from C-1. Substance P formed from [4-14C,1,2-3H2]cholecalciferol retained 36% of 3H, the amount expected from its distribution between C-1 and C-2, the 3H at C-1 being lost. 25-Hydroxycholecalciferol does not seem to have any specific intracellular localization within the intestine of rachitic chicks. The 3H-deficient substance P was present in the intestine and bone 1h after a dose of vitamin D and 30min after 25-hydroxycholecalciferol. There was very little 25-hydroxycholecalciferol in intestine at any time-interval, but bone and blood continued to take it up over the 8h experimental period. It is suggested that the intestinal 3H-deficient substance P originates from outside this tissue. The polar metabolite found in blood and which has retained its 3H at C-1 is not a precursor of the intestinal 3H-deficient substance P.