The activity of dihydrotachysterol and cholecalciferol was determined in nephrectomized or sham-operated vitamin D-depleted rats using in vitro transport of calcium and phosphate by everted intestinal preparations as the index of physiologic response. The activity of dihydrotachysterol was not reduced by absence of the kidneys whereas that of cholecalciferol was markedly inhibited so that at least a 10-fold greater dose of the latter was necessary to produce an equivalent effect in the nephrectomized rat as in the control. Dihydrotachysterol is therefore equipotent with cholecalciferol in the anephric rat although much less active in the intact animal.