25,26-Dihydroxyvitamin D3(25,26-dihydroxycholecalciferol) is a metabolite of vitamin D3 with intestinal calcium transport activity. IC50 value: Target: VD metabolite The biological activity of synthetic 24,25 and 25,26 diOHD3 was studied in vitamin D-deficient rats. The purpose of this study was to investigate the influence of small doses of both metabolites (0.125-0.250 mug) upon intestinal calcium transport and bone calcium mobilization. Both metabolites were able to increase calcium absorption in rats maintained on a calcium-deficient diet, but failed to do it in rats on a normal calcium diet. Bilateral nephrectomy suppressed this effect. The "bone calcium mobilization" of both derivatives was measured in vitamin D and calcium- or phosphorus-deprived rats after one intravenous dose. When serum calcium was initially low, 24,25 and 25,26 diOHD3 increased serum calcium moderately, but the increment was only significant with 24,25 diOHD3.