Objective: The aim was to examine the effects of aldosterone and of an aldosterone antagonist, spironolactone, on neointimal thickening in a rabbit model of balloon injury. Methods: Eighteen rabbits underwent aortic and iliac balloon injury and were randomised to subcutaneous infusion of aldosterone (70 μg·kg⁻¹·d^-t-1) or vehicle solution for 28 d. Eighteen other rabbits were randomised to receive daily subcutaneous injections of spironolactone (50 mg·kg⁻¹·d⁻¹) or of vehicle for 7 d before injury and for 28 d after the procedure. All animals were then killed just after measurement of plasma renin activity and of arterial pressure. Vessels were fixed and five cross sections were analysed per rabbit (three aortic; two from iliac artery). Mean values of neointimal area and of the neointimal area/medial area ratio were calculated. Results: Aldosterone treatment was associated with a decrease in renin activity and a non-significant increase in mean arterial pressure. Aldosterone significantly augmented the neointimal thickening in the iliac artery [0.42(SEM 0.07) v 0.24(0.03) mm², P < 0.05]but not in the aorta [0.63(0.08) v 0.59(0.12) mm², NS]. Spironolactone significantly inhibited intimal thickening, both in the iliac artery [0.09(0.02) v 0.29(0,01) mm², P < 0.001]and in the aorta [0.31(0.03) v 0.59(0.06) mm², P < 0.001]. Spironolactone administration was associated with an increase in renin activity and a decrease in mean arterial blood pressure. Conclusions: Aldosterone administration enhances neointimal thickening after injury and spironolactone, an aldosterone antagonist, is a potent inhibitor of neointimal thickening in the same model. This suggests a role for aldosterone in the pathophysiology of neointimal proliferation after balloon injury and for aldosterone antagonists in its prevention.