Poly(ADP-ribose) polymerase-1 (PARP-1), the most abundant member of the PARP family, is a nuclear enzyme that catalyzes ADP-ribose transfer from NAD⁺ to specific acceptor proteins in response to DNA damage. Excessive PARP-1 activation is an important cause of infarction and contractile dysfunction in heart tissue during interruptions of blood flow. The mechanisms by which PARP-1 inhibition and disruption dramatically improve metabolic recovery and reduce oxidative stress during cardiac reperfusion have not been fully explored. We developed a mouse heart experimental protocol to test the hypothesis that mitochondrial respiratory Complex I is a downstream mediator of beneficial effects of PARP-1 inhibition or disruption. Pharmacological inhibition of PARP-1 activity produced no deterioration of hemodynamic function in C57BL/6 mouse hearts. Hearts from PARP-1 knock-out mice also exhibited normal baseline contractility. Prolonged ischemia-reperfusion produced a selective defect in Complex I function distal to the NADH dehydrogenase component. PARP-1 inhibition and PARP-1 gene disruption conferred equivalent protection against mitochondrial Complex I injury and were strongly associated with improvement in myocardial energetics, contractility, and tissue viability. Interestingly, ischemic preconditioning abolished cardioprotection stimulated by PARP-1 gene disruption. Treatment with the antioxidant N-(2-mercaptopropionyl)-glycine or xanthine oxidase inhibitor allopurinol restored the function of preconditioned PARP-1 knock-out hearts. This investigation establishes a strong association between PARP-1 hyperactivity and mitochondrial Complex I dysfunction in cardiac myocytes. Our findings advance understanding of metabolic regulation in myocardium and identify potential therapeutic targets for prevention and treatment of ischemic heart disease.