Objectives: Degenerative mitral regurgitation (MR) is often accompanied by left ventricular (LV) hypertrophy and diastolic dysfunction. This study determined the contribution of diastolic dysfunction on LV remodeling and exercise capacity in rats with significant MR. Methods: MR was created in 8 Sprague-Dawley rats by following method. A fine needle (0.45mm diameter) was introduced through LV apex under the guidance of transesophageal echocardiography and made a hole on posterior mitral leaflet. MR was considered significant if a regurgitant jet area occupied more than 45% of the left atrium. In 4 rats (LVH+MR group), suprarenal aorta was ligated 2 weeks before MR formation. Echocardiography was performed at baseline, one week, and 2 weeks after MR formation. Exercise capacity was evaluated by swimming duration at week 2. Results: Baseline LV ejection fraction (EF) was 77짹6% and regurgitant jet area was 56짹8%. During the follow-up period, LV end-systolic and end-diastolic dimension and E/E' increased in both of MR and LVH+MR group (all, p<0.01, Figure) whereas EF did not change. Between the two groups, only E/E' was significantly higher in LVH+MR group than MR group at week 2 (21.7짹7.4 vs. 43.0짹2.5, p=0.03). Swimming duration was longer in MR group than LVH+MR group (158.3짹17.8 vs. 86.7짹28.9sec, p<0.05). Conclusions: LV hypertrophy and diastolic dysfunction badly affect LV remodeling and exercise capacity in MR. Evaluation of LV diastolic function might be important in patients with chronic MR.