The sites and modes of action of five groups of diuretics within the nephron, and their effects on magnesium excretion are outlined. More than 60% of the glomerularly filtered magnesium is reabsorbed in the loop of Henle. Hence, the loop diuretics induce a strong magnesium excretion. The benzothiadiazine diuretics increase magnesium excretion only moderately. In contrast, the potassium-sparing diuretics amiloride and triamterene, as well as the aldosterone antagonists, increase magnesium reabsorption and therefore may be designated as magnesium-conserving diuretic drugs. The possible modes of action of this property are discussed.