Heart rate variability (HRV) and echocardiography were performed in 14 dogs with mitral regurgitation (MR) before and after 14 days of 0.5mg/kg/day of enalapril treatment. All dogs were in heart failure stages B1 and B2. After enalapril treatment, left ventricular end diastolic diameter (LVEDd), left ventricular end diastolic diameter normalized for body weight (LVEDdN) and percent mitral regurgitant jet decreased (P<0.05). The diastolic blood pressure decreased (P<0.05). Increased time domain parameters of HRV were found. For frequency domain analysis, the total frequency (TF) increased significantly (P<0.05). The normalized low frequency (LF norm) decreased while normalized high frequency (HF norm) increased causing significant reduction in LF/HF (P<0.05). Before enalapril treatment, LF was correlated with end diastolic volume (EDV) (P<0.01) and LVEDd (P<0.05). In conclusion, MR dogs receiving enalapril treatment for 14 days had increased cardiac parasympathetic tone while sympathetic tone was suppressed. The decreased sympathetic activity corresponded to the reduction in cardiac preload and afterload.