The in vivo side-chain oxidation of 1.alpha.,25-dihydroxyvitamin D3 was investigated using a double-label radiotracer technique. Rats dosed with 1.alpha.,25-dihydroxy-[3.alpha.-3H]vitamin D3 and 1.alpha.,25-dihydroxy[26,27-14C]vitamin D3 produced compounds with a high 3H/14C ratio. The compounds were found in sizable quantities in intestine and liver within 3 h after dosing. The major side-chain oxidized metabolite migrated as an acid on DEAE-Sephadex chromatography and contained no 14C. Methyl esterification of this compound with diazomethane proceeded in good yield and rendered the compound more amenable to chromatographic purification. The metabolite was isolated in several steps from rats dosed with 1 .mu.g of 1.alpha.,25-dihydroxy[3.alpha.-3H]vitamin D3. The metabolite was obtained in pure form as the methyl ester and was positively identified as 1.alpha.,3.beta.-dihydroxy-24-nor-9,10-seco-5,7,10(19)cholatrien-23-oic acid. The trivial name calcitroic acid is proposed for this major side-chain oxidized metabolite of 1,25-dihydroxyvitamin D3.