F ollowing an I. V. calcium injection, there was a delayed return of the serum calcium to the basal levels in 17 patients with Graves' disease, 14 patients with simple goiter, and to patients totally thyroidectomized compared to 17 controIs. 111e results in 12 cases of toxic adenoma of the thyroid and seven persons after partial thyroidectomy (mainly lobectomy) did not differ significantly from the normals. F ollowing an I. V. injection of radioactive 47 Ca, the slope of the disappearance curve was faster in seven thyrotoxic patients compared to seven controIs. An I. V. injection of stable calcium chloride a<> celerated the slope in the normals but was without effect in the thyrotoxics. It is concluded that in cases with a diffuse disease process in the thyroid, such as Graves' disease or simple goiter, there is a deficient calcitonin release in response to provoked hypercalcaemia. Such a deficiency can not be proved in patients with a localized thyroid disease or fOllowing the surgical removal of a single thyroid lobe.