The authors observed that some patients being treated for postoperative tetany became insensitive to doses of vitamin D which had been adequate to maintain the plasma calcium levels and normal levels could no longer be maintained even with large doses of vitamin D. When a change was made from large doses of vitamin D to normal doses of A. T. 10 (dihydrotachysterol) or vitamin D3 the levels of calcium in the plasma could again be restored to normal. Thus there appeared to have developed an insensitvity to one form of vitamin D but not to closely related substances. This finding may have far-reaching implications in understanding other disorders such as vitamin D resistent rickets. The phenomenon appears to be unique as regards the action of vitamins.