Aberrant cardiac metabolism is linked to major health issues in the Western world including diabetes and heart failure. New tools are needed to investigate these conditions and to allow better diagnosis. In this work we used dissolution dynamic nuclear polarization NMR spectroscopy (dDNP-NMR) to investigate [1-13C]pyruvate metabolism in the isolated rat heart. A perfusion system simulating in vivo first-pass hemodynamics was used to measure the enzymatic flux through lactate dehydrogenase (LDH) using product selective excitation. LDH flux was found to be 52 ± 8 nmol lactate/s/g wet weight (n=3).