The effects of diuretics on renal handling of potassium and magnesium can result in depletion of these electrolytes. Associations between these deficits and the occurrence of increased ventricular ectopy raise the issue of potential relationships to increased risk of sudden unexpected death in hypertensive patients taking diuretics, especially patients with electrocardiographic and electrolyte abnormalities. Review of diuretic effects on the kidneys provides evidence that diuretic regimens that conserve potassium also conserve magnesium, and suggests the important role of these agents in protecting against potassium and magnesium abnormalities.