In this study we found that, in 31 normal subjects, close to 90% of circulating arginine vasopressin (AVP), measured by radioimmunoassay, was associated with platelets. By using routine methods of centrifugation, which do not completely separate platelets, the normal range of plasma vasopressin was higher by twofold than the normal range in platelet-free plasma prepared by differential centrifugation, which was 1.4 +/- 1.0 sd pg/ml. Platelet vasopressin was 12.9 +/- 5.7 pg/ml. Patients with congestive heart failure had, on average, an elevated platelet-free plasma AVP, as did two patients with thrombocytopenia and one with thrombocytosis. Patients with essential hypertension had slightly high levels of platelet-free plasma AVP and demonstrated an abnormal inverse relationship between platelet-free plasma AVP and serum osmolality. Immunoreactive platelet vasopressin was slightly low in patients with essential hypertension and was subnormal in patients with congestive heart failure. These studies demonstrate that platelets normally present in centrifuged plasma cause an overestimation of the plasma vasopressin levels. Until the physiological meaning of plasma and platelet-bound AVP is understood, studies of circulating vasopressin should probably assess both plasma and platelet AVP levels.