The roles of aldosterone in the progression of heart failure have not been fully elucidated. This study examined whether aldosterone nongenomically activates reactive oxygen species (ROS) production, causing myocyte apoptosis. Addition of aldosterone to neonatal rat cardiac myocytes caused the activation of NADPH oxidase and intracellular ROS production in a dose-dependent manner (10-(9)-10(-7) mol/L). NADPH oxidase activation was evident as soon as 5 min after aldosterone treatment. Neither an inhibitor for nuclear transcription (actinomycin D) nor an inhibitor of new protein synthesis (cycloheximide) blocked this rapid activation, and specific binding of aldosterone to plasma membrane fraction was inhibited by eplerenone, suggesting a nongenomic mechanism. Aldosterone did not affect the mRNA or protein levels of NOX2, which is a major subunit of NADPH oxidase in myocytes, after 48 h. Nuclear staining with DAPI showed that aldosterone (10(-7) mol/L) increased the myocyte apoptosis (2.3 fold, p<0.001), coincident with the activation of caspase-3 (1.4 fold, p<0.05), compared with the serum-deprived control after 48 h. Aldosterone also induced phosphorylation of apoptosis signal-regulating kinase 1 (ASK1). These effects of aldosterone on myocyte ROS accumulation, ASK1 activation, and apoptosis were abolished by eplerenone, a mineralocorticoid receptor (MR) antagonist, apocynin, an inhibitor of NADPH oxidase activation, and tempol, a free radical scavenger, but by neither RU486, a glucocorticoid receptor antagonist, nor butylated hydroxyanisol (BHA), a mitochondrial ROS scavenger. In conclusion, aldosterone-mediated ROS production is blocked by eplerenone and induced by the nongenomic activation of NADPH oxidase, leading to myocyte apoptosis associated with ASK1 activation. These proapoptotic actions of aldosterone may play a role in the progression of heart failure.