Abstract Body: Background: At birth, neonatal murine hearts transition abruptly from a hypoxic environment to oxygen-rich conditions, activating oxidative metabolism and increasing mitochondrial stress. However, how mitochondria coordinate stress adaptation while simultaneously developing their bioenergetic capacity during cardiac maturation remains poorly understood. Elucidating these mechanisms may reveal new therapeutic avenues to promote cardiac maturation and regeneration. Hypothesis: A coordinated mitochondrial energy and redox response help the newborn heart adapt to increased oxidative stress and supports postnatal maturation. Objective: To identify key mitochondrial stress-adaptations and define how they change during postnatal heart maturation. Methods: Whole hearts from C57BL/6J mice were harvested from P0–P14 (N=3). Mitochondria were isolated using sucrose–gradient centrifugation, and their function was evaluated with an ATP–coupled real–time respiration assay. Mitochondria biogenesis was evaluated by qPCR and immunoblotting. Whole–heart ROS levels (EPR signal/0.1 g tissue) were measured by EPR spectroscopy. Mitochondrial–enriched proteomic changes were analyzed by mass spectrometry. Results: Maturing neonatal hearts showed a significant increase in oxidative metabolism (p < 0.05), with the highest oxidative respiration observed at P17 (728.5 ± 20.1 pmol/min) compared with P6 (296.4 ± 12.7 pmol/min) and P2 (243.1 ± 25.9 pmol/min), correlating with enhanced mitochondrial biogenesis, reflected by mtDNA copy number increases of 4.2–fold at P6 and 8.6–fold at P14 relative to P2. ROS signal was significantly higher at P4 (33.7 ± 0.71/0.1 g) compared to P10 (26.9±2.8/0.1 g) or P14 (30.8±0.45/0.1 g), suggesting that postnatal metabolic reprogramming requires effective ROS buffering. Interestingly, Lonp1 level was lower at P2–P6 but increased 1.4–fold between P6–P14. This change coincided with a metabolic shift, characterized by upregulation of ETC subunits, TCA cycle enzymes, and FAO–related proteins, along with downregulation of glycolytic enzymes. Conclusion: Postnatal cardiac maturation involves coordinated increases in mitochondrial oxidative capacity, biogenesis, and redox adaptation, with Lonp1 likely playing a key role in these processes.