A 35-year-old woman experienced tetanic symptoms when treated with chorionic gonadotrophins or estrogenic oral contraceptives. Persistent hypocalcemia was found, with hyperphosphatemia, normal renal function and low normal plasma parathyroid hormone (PTH), all consistent with idiopathic hypoparathyroidism. During EDTA infusion, no PTH response was measured with a predominantly anti-NH2 -terminal antiserum, but a normal response was found with a predominantly anti-COOH-terminal antiserum. This supposes secretion of an immunologically abnormal and biologically ineffective PTH. Oral administration of ethinyl estradiol caused an impressive hypocalcemia with tetanic symptoms. Estrogens might, therefore, inhibit bone resorption by a specific action on bone, and not by antagonizing the action of PTH.