The metabolism of magnesium was compared with that of potassium in 5 patients with primary aldosteronism and in an adrenalectomized subject. In primary aldosteronism the urinary excretion and clearance of magnesium is increased although the serum magnesium is normal. A rapid infusion of isotonic saline resulted in a markedly increased excretion of both potassium and magnesium, probably reflecting an increased filtered load. After the surgical removal of adrenal adenomata there was a simultaneous decrease in both the urinary potassium and magnesium, which continued for several weeks after surgery. Spironolactone reduced the urinary magnesium in 2 patients with primary aldosteronism. The increase in both urinary potassium and magnesium after the administration of 400 μg d-aldosterone intramuscularly in an adrenalectomized patient was reversed by spironolactone. These studies suggest that aldosterone increases the clearance and excretion of magnesium and potassium in a parallel manner.