Backgound: Animal studies suggest that reactive oxygen species (ROS) play an important role in the development of diabetic cardiomyopathy. Hypothesis: Matrix metalloproteinase-2 (MMP-2) is activated by ROS and contributes to the acute loss of myocardial contractile function by targeting and cleaving susceptible proteins including troponin I (TnI) and α-actinin. Methods: Using the streptozotocin-induced diabetic rat model, we evaluated the effect of daily in vivo administration of sodium selenate (0.3 mg/kg; DMS group), or a pure omega-3 fish oil with antioxidant vitamin E (omega-3E; 50 mg/kg; DMFA group), which has antioxidant-like effects, for 4 weeks on heart function and on several biochemical parameters related to oxidant stress and MMP-2. Results: Although both treatments prevented the diabetes-induced depression in left ventricular developed pressure (LVDP) as well as the rates of changes in developed pressure (±dP/dt) (P<001), the improvement in LVDP of the DMS group was greater compared to that of the DMFA group (P<001). Moreover, these treatments reduced the diabetes-induced increase in myocardial oxidized protein sulfhydryl and nitrite concentrations (P<001). Gelatin zymography and Western blot data indicated that the diabetes-induced changes in myocardial levels of MMP-2 and tissue inhibitor of matrix metalloproteinase-4 (TIMP-4) and the reduction in TnI and α-actinin protein levels were improved in both the DMS and DMFA groups (P<001). Conclusions: These results suggest that diabetes-induced alterations in MMP-2 and TIMP-4 contribute to myocardial contractile dysfunction by targeting TnI and α-actinin and that sodium selenate or omega-3E could have therapeutic benefits in diabetic cardiomyopathy. © 2010 Elsevier Inc.