The malate-aspartate (M-A) shuttle provides an important mechanism of metabolic communication between the cytosol and the mitochondria. In this study, dynamic 13C NMR spectroscopy was combined with a multi-domain model of cardiacmetabolism for direct quantification ofmetabolic fluxes through the tricarboxylic acid (TCA) cycle (VTCA) and the M-Ashuttle (V M-A) in intact heart. The sensitivity of this approach to altered M-A shuttle activity was examined at different cytosolic redox states. Dynamic 13C NMR spectra were acquired from isolated rat hearts perfused with 13C labeled fatty acid at either low (fatty acid only) or high cytosolic redox state induced by exogenous glucose and lactate. VTCA and VM-A were determined by least-square fitting of the model to NMR data. Our results showed that while VTCA was similar, VM-A increased by 75% at high cytosolic redox state. Therefore, our proposed method provides the opportunity for direct quantification of metabolic communication between subcellular compartments via the M-A shuttle.