The renin-aldosterone system contributes to the regulation of arterial pressure and to the maintenance of sodium and potassium balance. Alterations in plasma potassium concentration have opposite and independent effects on renin secretion by the kidney and on aldosterone secretion by the adrenal gland. Renin secretion tends to be inhibited by hyperkalemia and stimulated by potassium depletion. In contrast, increases of plasma potassium directly stimulate aldosterone secretion. This effect of potassium on aldosterone serves as a protective mechanism against the development of hyperkalemia. Conversely, hypokalemia inhibits aldosterone production. Small changes in plasma potassium have a greater effect on aldosterone than on renin secretion. In patients with essential hypertension, diuretic induced alterations in serum potassium concentrations may affect both renin and aldosterone secretion. We have observed that therapy with a thiazide diuretic results in a reduction of serum potassium and a greater increase in renin activity than therapy with the potassium-retaining diuretic, spironolactone, despite comparable natriuretic responses with both drugs. Conversely spironolactone therapy is associated with a greater increase in aldosterone production. The greater effect of thiazides on renin activity and the greater effect of spironolactone on aldosterone production may be related to the thiazide induced reduction of serum potassium and the spironolactone induced increases of serum potassium.