This study examines the functional implications of postnatal changes in the expression of the mitochondrial transporter protein, 2-oxoglutarate-malate carrier (OMC). Online ¹³C nuclear magnetic resonance (¹³C NMR) measurements of isotope kinetics in hearts from neonate (3–4 days) and adult rabbits provided tricarboxylic acid cycle flux rates and flux rates through OMC. Neonate and adult hearts oxidizing 2.5 mM [2,4-¹³C₂]butyrate were subjected to either normal or high cytosolic redox state (2.5 mM lactate) conditions to evaluate the recruitment of malate-aspartate activity and the resulting OMC flux. During development from neonate (3–4 days) to adult, mitochondrial protein density in the heart increased from 19 ± 3% to 31 ± 2%, whereas OMC expression decreased by 65% per mitochondrial protein content (P < 0.05). Correspondingly, OMC flux was lower in adults hearts than in neonates by 73% (neonate = 7.4 ± 0.4, adult = 2.0 ± 0.1 μmol/min per 100 mg mitochondrial protein; P < 0.05). Despite clear changes in OMC content and flux, the responsiveness of the malate-aspartate shuttle to increased cytosolic NADH was similar in both adults and neonates with an approximate threefold increase in OMC flux (in densitometric units/100 mg mitochondrial protein: neonate = 25.8 ± 2.5, adult = 6.0 ± 0.2; P < 0.05). The¹³C NMR data demonstrate that OMC activity is a principal component of the rate of labeling of glutamate.