Spironolactone, a potent aldosterone-blocking agent, was administered to 34 subjects including 4 patients with primary aldosteronism, and changes in plasma renin activity and electrolytes were followed up. Plasma renin activity was found to elevate during the medication in all subjects except 4, who were suffering from primary aldosteronism. In the latter subjects there was no increase in the renin activity. A possibility was suggested with regard to the differential diagnosis of primary aldosteronism from other mimicking hypertensions using this spironolactone test.