In 1962 Copp et al. (1962) demonstrated a second, plasma calcium lowering factor from the parathyroid glands called calcitonin. These findings were subsequently confirmed by Kumar et al. (1963). Up to the present, however, calcitonin has not been demonstrated in man. In a patient with hypocalcemic tetany the injection of a commerical parathyroid extract caused the same changes in plasma calcium as described by Copp et al. (1962) in dogs, i.e. an initial fall and a subsequent rise in the calcium level. In addition, changes in plasma magnesium paralleling those of calcium were also observed.