Plasma aldosterone levels are reported to be lower in healthy elderly individuals compared with young individuals, a difference exaggerated by sodium depletion or upright posture. The aim of this study was to determine the aldosterone response to increases in serum potassium with advancing age. In the Clinical Research Center, six healthy young (20 to 35 yr of age) and six healthy elderly (65 to 85 yr of age) subjects underwent evaluation of their aldosterone responses to potassium infusion (0.5 mEq/kg over 45 min). Both young and elderly subjects had similar basal serum potassium levels (4.3 +/- 0.2 versus 4.4 +/- 0.1 mEq/L), similar sodium and potassium excretion amounts and similar increase in serum potassium levels during infusion (to 5.0 +/- 0.2 versus 5.1 +/- 0.1 mEq/L). However, elderly subjects had lower basal levels of plasma aldosterone and a blunted aldosterone response to potassium infusion (P < 0.05, analysis of variance). Advancing age is characterized by relative hypoaldosteronism in the basal state as well as in response to hyperkalemia. This may contribute to an increased susceptibility to hyperkalemia if other potassium regulatory systems fail.