Current guidelines recommend withdrawal of treatments that affect the aldosterone/renin ratio (ARR) when screening for primary aldosteronism (PA). However, abandonment of mineralocorticoid-receptor antagonist (MRA) and/or blockers of the renin-angiotensin system can deteriorate control of blood pressure (BP) and hypokalemia. Thus, in consecutive patients with an unambiguous diagnosis of PA in washout from confounding treatments and subtyped by AVS, we will compare within-patient plasma aldosterone and active renin concentration, and the ARR values, measured at baseline, and after a 1-month treatment with MRA alone and combined with an AT-1 receptor blocker (ARB). Patients on a regular salt intake will be treated with canrenone (50-100 mg orally) for 1 month, after which olmesartan (10 or 20 mg orally) will be added for another month with up-titration of both treatments over the first 2 weeks to control BP and hypokalemia, while background therapy will be maintained. The biochemical variables and the ARR will be assessed in an identical manner at baseline values and after each month of treatment. With a sample size of 40 patients, the study will have a 95% power to show a clinically significant 20% change in the ARR at a 5% α value using a two-sided paired t test. Hence, this study will allow us to determine if an MRA alone, or added to an ARB at doses that control BP and hypokalemia, affects or not the ARR, thus allowing to establish if these agents can be administered or must be forbidden during the screening of PA.