Thiazide therapy is a widely used first line treatment for arterial hypertension. Its useful value, particularly in mild or moderate hypertension, is sometimes reduced by metabolic side-effects, as hypokalaemia and hyperuricaemia. In the present study the antihypertensive efficacy of a new, non-sulphonamide diuretic Bay g 2821 (muzolimine) was evaluated in comparison with the combination of hydrochlorothiazide-amiloride over a period of 4 weeks. A highly significant decrease in systolic and diastolic blood pressures was produced by both treatments. No decrease in serum potassium nor an increase in cholesterol, triglycerides, uric acid or glucose was detected during the 4 week treatment period. Subjective side-effects, such as headache and dizziness, were very rarely observed during Bay g 2821 treatment. The new diuretic appears, therefore, to be effective in the treatment of arterial hypertension without untoward side-effects.