Heart failure develops when compensatory mechanisms fail to overcome injury and hemodynamic stress that can be caused by myocardial infarction, pressure and volume overload, or hypertension. Increased stress on the heart results in a reorganization of the myocardial extracellular collagen matrix (ECM), leading to fibrosis and dysfunction. Lysyl oxidase (LOX) is critical for the enzymatic cross-linking necessary for normal collagen processing, yet over-activation of LOX is associated with fibrosis. LOX is significantly elevated in failing human hearts, but it is not known if excess LOX activity accelerates cardiac failure. Using a rodent model of heart failure, we assessed the role of LOX activity in the progression of cardiac dysfunction and adverse ECM alterations. Statistical analyses were performed using one-way ANOVA with Dunnett’s post test, and considered significant at p<0.05. Surgically induced volume overloaded and sham-operated rats were studied at 14 wks post-surgery (n=5 to 7 per group). To assess the role of LOX in disease progression, rats were treated with a LOX inhibitor (BAPN; 100 mg/kg/d) starting 2 wks post-surgery. LOX inhibitor or vehicle saline was delivered using Alzet osmotic minipumps implanted in the abdominal cavity. Echocardiography was used to evaluate cardiac function and progression of left ventricular (LV) remodeling. LV LOX expression and activity were determined by Western blot and Amplex red assay, respectively. Extracellular collagen assays included: interstitial fibrosis by picrosirius red staining of LV sections; cross-linking by pyridinoline content; and collagen type I and III expression by western blot. Volume overload (VO) caused significant ventricular dilatation (43% increase) and dysfunction (26% decrease, %FS) by 14 wks post-surgery versus Sham. LOX protein expression was increased (65%; p<0.05 vs Sham) by VO with concomitant increases in LOX activity. These increases in LOX were associated with significantly elevated interstitial collagen, cross-linking, and type I/III ratio (p<0.05 vs Sham). LOX inhibition prevented the development of cardiac dysfunction and blocked the VO-induced changes in LV collagen. LOX inhibition normalized LV collagen staining, cross-linking, and expression to levels comparable to age-matched Sham. Our findings indicate that LOX inhibition was cardioprotective in the volume overload stressed heart. Further studies are warranted to determine the cellular mechanisms responsible for increased LOX expression and LOX-dependent damage in the stressed heart.