Twelve chronic in-patients with primary polydipsia were studied, during free drinking and after fasting, by concurrent measurements of plasma AVP, serum sodium and osmolality, and urine volume, AVP, osmolality, and creatinine. A majority of the patients showed inappropriately high levels of AVP: plasma AVP estimations demonstrated that seven had Type I SIADH and two had Type II SIADH. Urinary AVP estimations confirmed inappropriately raised AVP in seven of the subjects tested, and there was a significant agreement between the plasma and urine diagnoses. Although able to concentrate their urine in response to fluid deprivation, the patients showed a decreased renal sensitivity to AVP. Despite the mitigating effect of decreased renal sensitivity to AVP, the SIADH seen in these patients appears to contribute to the development of water intoxication caused by polydipsia.