The infusion of 40 mEq potassium (aspartate) in 250 ml isotonic 1-fructose at a rate of 20 mEq/h into 5 patients (34-56 years old) with aldosteronoma and 2 patients with bilateral primary aldosteronism consistently raised their mean arterial pressure by 15-20 mmHg. Their pressure values returned to the baseline levels 4-5 h after the infusion. In contrast, in controls (10 patients with idiopathic arterial hypertension, matched for age, sex, and magnitude of the untreated hypertension, and 7 patients with inactive adrenal nodules as incidental findings on upper abdomen ultrasound or computerized tomography) the same procedure caused negligible arterial pressure changes. The cause of the rise in blood pressure observed uniquely in patients with primary aldosteronism after infusion of potassium (aspartate) cannot be accounted for by an increase in plasma aldosterone, blood volume, or plasma angiotensin II. The cause of this response thus remains obscure; nonetheless, this simple procedure may prove useful in differentiating primary aldosteronism from idiopathic hypertension, in excluding the adrenal disorder, and in revealing even its mildest forms.