It is currently said that CO has anti-inflammatory and antiapoptosis effects and it has attracted attention as a medical gas. We used CO for rat hearts and conducted a preservation experiment. We isolated rat hearts, placed them into a specially made chamber, filled the chamber with a gas mixture of PCO (4,000 hPa) and PO(2) (3,000 hPa), and preserved the hearts in a refrigerator at 4°C for 48 h. We then performed a heterotrophic transplantation on the neck of each recipient rat and resuscitated the preserved hearts. We herein report our findings.