SUMMARY1. Changes in plasma active and inactive renin concentration (ARC and IRC) after captopril administration and angiotensin II (AII) infusion were studied in six patients with Bartter's syndrome.2. A single oral dose of captopril (8‐25 mg) lowered the blood pressure and increased both ARC and IRC. AII infusion elevated blood pressure, suppressed ARC and increased IRC.3. In this syndrome of high renin levels, infused AII appeared to increase inactive renin secretion by reducing its conversion to active renin. On the other hand, an acute fall in AII levels and/or renal perfusion pressure by captopril increased both active and inactive renin. This indicates that the increase in the secretion of inactive renin, stimulated by captopril, might exceed any increase in its conversion to active renin in patients with Bartter's syndrome, in whom the production of renin is accelerated, and conversion of inactive renin to active renin probably already operates near its maximu