Oral administration of potassium chloride to normal subjects and patients with primary aldosteronism had no effect on plasma renin activity. When potassium chloride was given orally to patients with renovascular hypertension, plasma renin activity decreased markedly as compared with the control values. Moreover, an increase in plasma renin activity induced by sodium restriction or diuretics ad-ministration to normal volunteers was prevented by potassium supplement. From these findings it appears that potassium loading suppresses renin release by the kidney. This suppressive effect of potassium on renin release would possibly be mediated through the feedback mechanism of increased aldosterone secretion produced directly by potassium administration.