Angiotensin converting enzyme (ACE) inhibitors represent the front line pharmacological treatment of heart failure, which is characterized by left ventricular (LV) dilatation and inappropriate hypertrophy. The mechanism of action of ACE inhibitors is still unclear, but evidence suggests that they may act by influencing matrix metalloproteinase (MMP) activity. This study sought to determine whether ACE inhibitors can directly regulate MMP activity, and whether this results in positive structural and functional adaptations to the heart. To this end, MMP activity in LV tissue extracted from rats with an aortocaval (AV) fistula was assessed by in vitro incubation as well as in vivo treatment with captopril, lisinopril or quinapril. Further, LV size and function were determined in untreated AV fistula rats, AV fistula rats treated with lisinopril (3, 5 and 8 weeks) and age-matched sham-operated controls. In vitro incubation with captopril, lisinopril or quinapril significantly reduced MMP activity, as did in vivo treatment. This occurred without a reduction in the available pool of MMP-2 protein. Long-term in vivo administration of lisinopril also prevented LV dilatation, attenuated myocardial hypertrophy and prevented changes in myocardial compliance and contractility. The results herein demonstrate that ACE inhibitors prevent MMP activity, and in so doing represent a mechanism responsible for preventing the negative structural and functional changes that occur in the rat AV fistula model of heart failure.