Accumulation of oxidized extracellular matrix between endothelium and muscle is an important risk factor in the endothelium-myocytes uncoupling in congestive heart failure. Although ventricular remodeling is accompanied by increased matrix metalloproteinase (MMP)-9 activity, it is unclear whether MMP-9 plays a role in endothelial apoptosis in chronic volume overload congestive heart failure. We tested the hypothesis that, in chronic volume overload, myocardial dysfunction involves endocardial endothelial (EE) apoptosis in response to MMP-9 activation, extracellular matrix accumulation, and endothelium-myocytes uncoupling. Arteriovenous fistula (AVF) was created in control (FVB/NJ) and MMP-9 knockout (MMP-9KO; FVB.Cg-MMP9(tm1Tvu)/J) mice. Sham surgery was used as control. Mice were grouped as follows: wild type, n = 3 (sham control); MMP-9KO, n = 3 (sham); AVF, n = 3; and MMP-9KO + AVF (n = 3). Heart function was analyzed by M-mode and Doppler echocardiography, and with a pressure-tipped Millar catheter placed in the left ventricle of anesthetized mice 8 wk after AVF. Apoptosis was detected by measuring caspase-3, transferase-mediated dUTP nick-end labeling (TUNEL), and CD-31 by immunolabeling. Protease-activated receptors-1, connexin-43, and a disintegrin and MMP-12 (ADAM-12) expression were measured by Western blot analyses. MMP-2 and MMP-9 expression were measured by quantitative RT-PCR. Compared with control, AVF caused an increase in left ventricle end diastolic pressure and decrease in -dP/dt. In contrast, in the MMP-9KO + AVF group, these variables were changed toward control levels. Increased EE apoptosis (caspase-3 activation and TUNEL/CD-31 colabeling) in AVF mice was prevented in the MMP-9KO + AVF group. Protease-activated receptor-1, connexin-43, and ADAM-12 were induced in AVF. MMP-9 gene ablation ameliorated the induction. The results suggest that impaired cardiac function in volume overload is associated with EE apoptosis, cardiac remodeling, and endothelium-myocytes uncoupling in response to MMP-9 activation.