Clinical Chemistry 46, No. 10, 2000 1699 peripherally and centrally regulated functions such as antidiuresis and blood pressure regulation in peripheral tissues and temperature regulation in the central nervous system (1). The antidiuretic function of AVP controls the serum osmolality by stimulating renal water resorption (1, 2). In patients with FNDI, there is insufficient release of AVP, leading to insufficient stimulation of renal water resorption (1, 3–5). Often, these patients have a constant feeling of thirst that leads to an excessive water intake to compensate for the high urinary output. FNDI is currently diagnosed by means of the patientunfriendly water dehydration test in which the patient is deprived of water intake for a certain period. In the case of vasopressin deficiency, there will be continuing water loss, producing weight loss and hypertonic dehydration (4), which can be dangerous if the patient is not carefully observed during the test.