Objective: The aim of this study was to test the hypothesis that, shortly after the development of Type-2 diabetes, alterations in cardiac carbohydrate metabolism precede the onset of abnormalities in systolic function. Methods: Hearts from 11-week-old Zucker diabetic fatty (ZDF) rats and age matched controls were perfused in the isovolumic Langendorff mode with 13C-labeled glucose, lactate and pyruvate and unlabeled fatty acids. 13C-Nuclear magnetic resonance glutamate isotopomer analysis was carried out to determine the contributions of substrates to energy production. Results: The ZDF group was hyperglycemic and the relative flux through pyruvate dehydrogenase (PDH) was significantly depressed compared to lean controls. In the lean group, lactate, pyruvate and glucose contributed 64±3, 24±3 and 11±1%, respectively, to total pyruvate oxidation. In the ZDF group, the contribution of glucose both to total pyruvate oxidation and to tissue lactate and alanine formation was significantly depressed. Cardiac function assessed by the rate-pressure product was similar in both groups. The fraction of active PDH was decreased in the ZDF group compared to controls (p<0.025). Conclusions: These results highlight significant changes in cardiac carbohydrate metabolism shortly after the development of hyperglycemia in a model of Type 2 diabetes in the absence of overt changes in systolic function. © 2002 Elsevier Science B.V. All rights reserved.