The plasma arginine vasopressin (AVP) level was measured by RIA in 19 patients with polyuria and polydipsia, and the AVP measurement was compared with indirect tests. The AVP levels were very low in 13 patients with central diabetes insipidus and normal in 6 patients with primary polydipsia. The direct assay was correspondent to the indirect criteria in 89% of the patients. We concluded that the use of AVP assay improves accuracy in the differential diagnosis of polyuria, and is important in the diagnosis of these diseases.