The effect of five days' regimen with a low-sodium diet (approximately 20 mmol Na+ per day) and with 40 mg of furosemide (Lasix) daily on the blood pressure, the renin-aldosterone system and the electrolyte balance was studied in 42 hypertensive patients who received no antihypertensive drugs for four weeks prior to the investigation. Twenty-three of the patients had renal artery stenosis. Both patients with and those without such stenosis showed a significant decrease in systolic and diastolic blood pressure and in the serum concentrations of sodium and potassium and urinary excretion of sodium. A concomitant increase in plasma renin activity in peripheral veins and in urinary excretion of aldosterone was found both in patients without and in those with renal artery stenosis, but in the latter group this response seemed to be less pronounced.