AbstractInhibition of succinate dehydrogenase (SDH) by Dimethyl Malonate (DiMal) reduces cardiac ischemia–reperfusion (IR) injury. We investigated the cardioprotective effect of DiMal in a rat model during advancing type 2 diabetes. Zucker Diabetic Fatty rats and lean controls were investigated corresponding to prediabetes, onset and mature diabetes. Hearts were mounted in an isolated perfused model, and subjected to IR for investigation of infarct size (IS) and mitochondrial respiratory control ratio (RCR). DiMal was administered for 10 min before ischemia. Compared with age-matched non-diabetic rats, prediabetic rats had larger IS (49 ± 4% vs. 36 ± 2%, p = 0.007), rats with onset diabetes smaller IS (51 ± 3% vs. 62 ± 3%, p = 0.05) and rats with mature diabetes had larger IS (79 ± 3% vs. 69 ± 2%, p = 0.06). At the prediabetic stage DiMal did not alter IS. At onset of diabetes DiMal 0.6 mM increased IS in diabetic but not in non-diabetic control rats (72 ± 4% vs. 51 ± 3%, p = 0.003). At mature diabetes DiMal 0.1 and 0.6 mM reduced IS (68 ± 3% vs. 79 ± 3% and 64 ± 5% vs. 79 ± 3%, p = 0.1 and p = 0.01), respectively. DiMal 0.1 mM alone reduced IS in age-matched non-diabetic animals (55 ± 3% vs. 69 ± 2% p = 0.01). RCR was reduced at mature diabetes but not modulated by DiMal. Modulation of SDH activity results in variable infarct size reduction depending on presence and the stage of diabetes. Modulation of SDH activity may be an unpredictable cardioprotective approach.