Objectives. This study examined the effects of early long-term monotherapy with enalapril on the severity of functional mitral regurgitation in dogs with moderate heart failure. Background. Functional mitral regurgitation often develops in patients with heart failure and, depending on its severity, can have a marked adverse impact on the stroke output of the failing left ventricle and contribute to progressive deterioration of the heart failure state. Methods. Left ventricular dysfunction (ejection fraction 30% to 40%) was produced in 14 dogs by multiple sequential intracoronary microembolizations. Dogs were randomized to 3 months of therapy with enalapril (10 mg twice daily, n = 7) or no therapy at all (control, n = 7). The severity of functional mitral regurgitation was quantified by Doppler color flow mapping in seven control and six enalapril-treated dogs. Mitral annular diameter was assessed by echocardiography and left ventricular volumes and shape by ventriculography. Measurements were made before initiation and after completion of therapy. Results. In control dogs, the severity of mitral regurgitation increased during the follow-up period ([mean ± SEM] 14 ± 4 vs. 23 ± 4%, p < 0.001) and was associated with increased left ventricular end-systolic and end-diastolic volumes. In contrast, the severity of regurgitation was not significantly changed in dogs treated with enalapril (18 ± 3 vs. 16 ± 6%, p < 0.59) and was associated with preservation of left ventricular volumes. Conclusions. In dogs with moderate heart failure, early long term therapy with enalapril prevents progressive worsening of functional mitral regurgitation. This beneficial effect is most likely achieved by prevention of progressive left ventricular dilation.