Seven patients with Bartter's syndrome were investigated before and after 3 months' treatment by enalapril. Serum potassium rose from 2.4 +/- 0.5 to 3.9 +/- 0.6 mmol/L. In all patients, serum magnesium rose and bicarbonate fell. Hormonal changes were as suspected: a further stimulation of renin and a decline in aldosterone. The BP sensitivity to angiotensin II normalized in the five patients in whom the test was performed. Clearance studies during maximal water diuresis, performed in four patients, were compatible with a high proximal fractional tubular sodium reabsorption and a relatively low distal fractional sodium reabsorption. Fractional free water excretion after furosemide was also low, confirming the concept of a primary sodium reabsorption defect in the furosemide-insensitive part of the nephron in Bartter's syndrome. The only consistent change after enalapril was a further decline in distal fractional sodium reabsorption. Initiation of therapy produced a BP fall in each subject. Clinical important hypotension associated with oliguria was seen twice, but these reactions were short-lasting. The BP rose to pretreatment values within 72 hours, despite continuation of converting-enzyme inhibition. Renal function recovered, though a moderate fall in function persisted. No other side effects were noticed. We conclude that converting-enzyme inhibition improves the potassium metabolism of patients with Bartter's syndrome, without ameliorating the abnormal renal sodium handling.