The effect of captopril on the aldosterone response to potassium was studied in 6 patients with idiopathic adrenal hyperplasia (IAH) and in 4 patients with an aldosterone-producing adenoma (APA), who all have suppressed activity of circulating renin-angiotensin system (RAS). Precaptopril, KCl infusion induced a significant increase in aldosterone in both groups. This increment was significantly blunted by captopril in IAH, but not in APA. Decreased potassium-stimulated aldosterone secretion after captopril in IAH supports the hypothesis that adrenal RAS plays a role in aldosterone production under potassium stimulation.