Abstract Body (Do not enter title and authors here): Background: Cardiovascular diseases, the leading cause of mortality with 17.9 million deaths annually (WHO), often necessitate heart transplants for end-stage organ failure. However, donor heart demand far exceeds supply. Traditional organ preservation methods like static cold storage (SCS) offer a limited 4-6 hour window, while machine perfusion (MP) extends this by circulating an oxygen-rich solution but faces challenges with nutrient depletion and metabolic imbalances. Our "partial freezing" method offers a novel approach to organ preservation using cryoprotectants (CPAs), by achieving a stable frozen state while maintaining an unfrozen fraction to limit ice damage and dehydration Hypothesis: Partial freezing (PF) method can effectively extend the storage duration of rodent hearts while maintaining their viability and function. Methods: The PF of the heart entails six major steps: (1) Procuring hearts (N=3) from adult male Lewis rats (2) Preloading of CPAs (DMSO, glycerol, raffinose, mannitol, and proline) during hypothermic Langendorff mode perfusion for 30 min (3) Freezing the hearts at a controlled rate of 0.3°C/min to -6°C (4) Storage at -6°C for 18 hours (5) Gradual thawing and unloading of CPAs (6) Recovery of hearts during NMP (normothermic machine perfusion) with antioxidant-supplemented Tyrode solution, followed by viability assessment using metabolic, hemodynamic, and molecular parameters. Two control groups were also included in the study: (1) Fresh hearts (N=3) functionally recovered in Tyrode solution, and (2) SCS hearts (N=3), perfused with cold University of Wisconsin (UW) solution and stored in the UW solution for 18 hours at 4°C, then recovered in same solution as the PF hearts. Results: NMP assessments revealed promising outcomes for PF hearts, with histology, TTC (Triphenyltetrazolium chloride) and Connexin-43 immunostaining profiles comparable to fresh controls and superior to SCS hearts. Although PF hearts exhibited lower beating rates, they demonstrated comparable metabolic parameters (lactate and oxygen consumption) and tissue edema compared to fresh controls, indicating preserved cardiac performance post-recovery. Conclusion: Our study shows the potential of the PF method to extend storage to 18 hours while maintaining viability and molecular function similar to fresh controls and superior to SCS hearts. This exploratory study suggests PF as a promising method to extend heart preservation times, addressing current limitations.