The level of serum magnesium and also thyroid of calcitonin content was followed up in the conditions of adrenalectomy after hydrocortisone replacement as well as after calcium propranolol administration. Six days after adrenalectomy a significant decrease both of calcitonin secretion and of serum magnesium was noted. Hydrocortisone replacement reestablished the levels of both parameters to the levels found in controls. Adrenalectomy also reduced the calcitonin secretory response consequent to calcium loading, but calcium excess simultaneously determined hypermagnesemia. Propranolol, a beta blocker with direct action on the secretory tonus of the thyroid C cells, completely annulled the stimulation effect of calcium in adrenalectomized animals but not the hypermagnesemic response. The adrenocortical hormones interfere in the calcitonin-magnesium secretory antagonism, together with the sympathetic -adrenergic system, ensuring the secretory control of calcitonin especially by blocking the occurrence of an excess of magnesium which reduces calcitonin secretion.