The effects of the angiotensin converting enzyme inhibitors benazepril (13), enalapril (E), and ramipril (R) on the function, geometry, and dimensions of the left ventricle (LV) in the dogs with naturally acquired severe mitral regurgitation (MR) were evaluated by echocardiography. Thirty-two dogs, aged 8-16 years and weighing 10-12 kg, with fractional shortening (FS) >50% were included into the Study. Benazepril (0.5mg/kg/d), enalapril (0.5mg/kg/d), and ramipril (0.5mg/kg/d) were administered orally to B, E, and R groups, respectively. Furosemide (2mg/kg/d) was administered orally for conventional heart failure therapy. Physical, radiographic, electrocardiographic, and echocardiographic examinations were performed before treatment and oil day 7 after the treatment. A decrease in the left ventricle end diastolic diameter (LVEDd) in groups E and R and in end diastolic volume (EDV) in group E was considerable during the subacute period (P<0.05). The stroke volume (S V) significantly decreased ill groups E and R (P<0.01), whereas group B dogs had a mild decrease (P<0.05). FS and ejection fraction (EF) that were higher before treatment decreased significantly in all groups after the treatment (P<0.001). Differences in SV, FS, and EF were not statistically significant between groups. It was detected that LV was remodelled as a result of the effects of enalapril and ramipril on LVEDd and also EDV was decreased by enalapril. Furthermore, it was observed that these ACE inhibitors were effective on geometry, dimensions, and functions on LV of the dogs with severe MR, and enalapril was found to be the most effective agent, followed by ramipril and benazepril.