• Despite the importance of disturbances in plasma volume in nephrotic patients, the only clinically accepted measurement of this value in common use is plasma renin activity. We assessed the usefulness of plasma vasopressin levels as an index of plasma volume in patients with idiopathic nephrotic syndrome. However, since 80% to 90% of arginine vasopressin circulates bound to platelets, we measured vasopressin levels in platelet-rich and platelet-poor plasma. The nephrotic patients (n=19) had significantly higher vasopressin levels in platelet-poor and platelet-rich plasma compared with controls 3.2±0.6 vs 1.0±0.3 pg/mL, and 10.4±3.6 vs 3.3±0.6 pg/mL, respectively). The percent binding of vasopressin to platelets was reduced in nephrotic patients compared with controls 50.2%±6% vs 70.4%±2.9%). The values for platelet-poor vasopressin, but not platelet-rich vasopressin, correlated significantly with the plasma renin activity (r=.83). We conclude that in nephrotic patients, platelet-poor vasopressin levels correlate with plasma renin activity and may provide a useful measure of minute-to-minute vasopressin release in response to changes in plasma volume.