Atrial natriuretic peptide (ANP) has been shown to suppress aldosterone secretion under certain circumstances, although the physiological significance of this is uncertain. We wondered if ANP would suppress potassium-stimulated aldosterone secretion in man and, if so, whether we might find high circulating levels of ANP in patients with the syndrome of acquired hypoaldosteronism. We studied seven healthy young subjects under two conditions: 1) infusion of KCl (0.5 mmol/kg) over 45 min, and 2) KCl infused with ANP (0.01 microgram/kg.min) for 60 min. We also evaluated ANP levels in eight elderly subjects with the syndrome of acquired hypoaldosteronism, as defined by hyperkalemia (mean serum K+, 5.3 +/- 0.1 mmol/L) associated with inappropriately low aldosterone levels (216 +/- 50 pmol/L). In the normal subjects, ANP almost completely suppressed the aldosterone response to KCl infusion (P less than 0.001, by analysis of variance) despite a similar rise in the serum potassium level with KCl alone (0.70 +/- 0.07 mmol/L) and KCl plus ANP (0.75 +/- 0.09 mmol/L). PRA fell slightly during KCl plus ANP treatment, but did not change during the infusion of KCl alone. ANP levels were approximately 800 pmol/L during the ANP infusion studies. Endogenous ANP levels in the hyperkalemic patients with hypoaldosteronism were markedly elevated at 1186 +/- 340 pmol/L (compared to 93 +/- 10 pmol/L in healthy elderly controls), a level that would be capable of suppressing the potassium-mediated aldosterone response. Exogenous infusion of ANP suppressed the aldosterone response to hyperkalemia, and ANP levels were found to be markedly elevated in a group of patients with hyperkalemia and hypoaldosteronism. We suggest that ANP may contribute to clinically significant hypoaldosteronism and hyperkalemia in the syndrome of acquired hypoaldosteronism.