Blunted -adrenergic inotropism in stunned myocardium is restored by pharmacological (N-acetylcysteine) and metabolic (pyruvate) antioxidants. The ketone body acetoacetate is a natural myocardial fuel and antioxidant that improves contractile function of prooxidant-injured myocardium. The impact of acetoacetate on post-ischemic cardiac function and -adrenergic signaling has never been reported. To test the hypothesis that acetoacetate restores contractile performance and -adrenergic inotropism of stunned myocardium, post-ischemic Krebs-Henseleit perfused guinea-pig hearts were treated with 5 mM acetoacetate and/or 2 nM isoproterenol at 15-45 and 30-45 min reperfusion, respectively, while cardiac power was monitored. The myocardium was snap-frozen and its energy state was assessed from phosphocreatine phosphorylation potential. Antioxidant defenses were assessed from GSH/GSSG and NADPH/NADP⁺ redox potentials. Stunning lowered cardiac power and GSH redox potential by 90% and 70%, respectively. Given separately, acetoacetate and isoproterenol each increased power and GSH redox potential three-to-fivefold. Phosphocreatine potential was 70% higher in acetoacetate vs. isoproterenol treated hearts (P<0.01). In combination, acetoacetate and isoproterenol synergistically increased power and GSH redox potential 16- and 7-fold, respectively, doubled NADPH redox potential and increased cyclic AMP content 30%. The combination increased cardiac power 4-6 foldvs. the individual treatments without a coincident increase in phosphorylation potential. Potentiation of isoproterenol's inotropic actions endured even after acetoacetate was discontinued and GSH potential waned, indicating that temporary enhancement of redox potential persistently restored -adrenergic mechanisms. Thus, acetoacetate increased contractile performance and potentiated -adrenergic inotropism in stunned myocardium without increasing energy reserves, suggesting its antioxidant character is central to its beneficial actions.