A group of 13 male and 8 female out-patients (mean age: 40.57 years), with primary arterial hypertension, was submitted to two periods of treatment in a cross-over study with spironolactone and potassium canrenoate. Each preparation was given at the dose of 200 mg/day per os for 21 days, with a 10-day interval between treatments. Both preparations proved active on the systolic and diastolic pressure values controlled 7 days. However, potassium canrenoate showed a greater and more rapid effect, particularly on diastolic arterial pressure, as also demonstrated by the statistical analyses. PRA and aldosteronemia increased with both treatments, but this increase resulted significantly lower with potassium canrenoate. The minor stimulating action on the renin-angiotensin-aldosterone mechanism by potassium canrenoate may be the reason for its greater anti-hypertensive effect. Both treatments were perfectly tolerated locally and systemically.