Abstract-Aldosterone has rapid nongenomic effects in the human vasculature. However, data are not uniform and little is known about chronic effects of aldosterone. Therefore, we investigated acute and chronic effects of elevated aldosterone levels on endothelial function in the forearm vasculature of healthy men. In a first crossover study, the effects of arterial aldosterone infusion in ascending doses (3.3 to 55 pmol/min per 1000 mL forearm volume) on forearm blood flow were investigated in 8 healthy men (26Ϯ2 years). In a second study, endothelium-dependent (acetylcholine; 0.08, 0.275, and 2.75 mol/min per 1000 mL) and endothelium-independent (sodium nitroprusside 0.02 mol/min per 1000 mL) vasodilation and basal nitric oxide formation (forearm blood flow response to blockade by N G -monomethyl-L-arginine 8 mol/min per 1000 mL) were tested in 10 healthy men (age 30Ϯ5 years) at baseline, during infusion of 55 pmol/1000 mL per min aldosterone (acute effects), and after 0.3 mg/d oral fludrocortisone for 2 weeks (chronic effects) on separate days. Forearm blood flow was assessed by venous occlusion plethysmography. No change in forearm blood flow was seen with aldosterone infusion alone. Acute coinfusion of aldosterone increased vasodilation to sodium nitroprusside by 93% (PϽ0.01) and to acetylcholine by 60% (Pϭ0.14). Response to N G -monomethyl-L-arginine did not change. After 2 weeks of oral fludrocortisone, response to acetylcholine was enhanced by 72% compared with baseline (Pϭ0.03). Additionally, response to N G -monomethyl-L-arginine was enhanced by 80% compared with baseline (Pϭ0.05). Aldosterone acutely enhances vasodilation to exogenous nitric oxide whereas mineralocorticoid excess for 2 weeks enhances basal nitric oxide bioactivity and improves endothelium dependent, nitric oxide-mediated vasodilation in the forearm vasculature of healthy men. Key Words: aldosterone Ⅲ endothelial function Ⅲ mineralocorticoid excess Ⅲ forearm vasculature Ⅲ nitric oxide A ldosterone has been claimed to lead to endothelial dysfunction (review, see Brown 1 ), a condition related to development of cardiovascular disorders and to poor prognosis. 2 However, studies of aldosterone effects on endothelial function led to discrepant findings, which may be related, at least in part, to inhomogeneity of the populations studied. Thus, studies in healthy subjects showed no detrimental effects of aldosterone on endothelial function and no positive effect of aldosterone inhibition, 3-5 whereas populations with established cardiovascular diseases showed negative effects of aldosterone and positive effects of spironolactone therapy. 6 -11 Still, other factors may be of importance as effects of aldosterone on endothelial function are not homogenous even in a healthy population. 12 Dosages of aldosterone, 3,4,6 concomitant drug use, 13 as well as the vascular bed investigated 5,14 -16 may influence the effects observed.Furthermore, little is known about chronic endothelial effects of aldosterone that could indicate a primary and direct role...