The function of the parathyroid glands was assessed in 5 hypothyroid patients, 5 hyperthyroid patients and 13 controls before as well as during and after an infusion of disodium EDTA. Serum calcium levels are significantly higher in hyperthyroid patients than in hypothyroid patients, normal controls having intermediary values. Serum phosphorus values, both in basal conditions and during or after EDTA infusion, are increased in hyperthyroid patients compared with hypothyroid or control patients. Serum parathyroid hormone levels, measured with two different antisera, are significantly lower in hyperthyroid than in hypothyroid patients. It is suggested that functional hypoparathyroidism prevails in hyperthyroidism, while parathyroid function is normal or increased in myxedema.