Purpose—To study mononuclear magnesium and serum cations (Na, K and Mg) in elderly hypertensive patients treated with hydrochlorothiazide during 90 days. Patients and Methods— 15 elderly hypertensive patients treated with hydrochlorothiazide, 25 mg/day or placebo. A method of freezing produced total Iysis of the cells; Mg was measured by atomic spectrophotometry. Results—No differences were noted in mononuclear or serum magnesium in the thiazide or placebo treated patients, and the diuretic produced significant decreases in supine systolic and diastolic blood pressure. Conclusion—Magnesium supplementation does not seem essencial in most patients in this conditions