Mitochondrial dynamics are altered in favor of mitochondrial fission during myocardial I/R. Mitochondrial fission is associated with shortening of mitochondria, decreased ATP production, and increased reactive oxygen species during I/R, and is thought to promote cardiomyocyte loss. Therefore, inhibition of mitochondrial fission may be a new strategy to salvage injured cardiac myocytes during I/R and limit infarct size. To test this hypothesis, the cardioprotective effects of a novel mitochondrial fission inhibitor, P110 (MW=2427 g/mol), a cell permeable peptide, that selectively inhibits the interaction between dynamin related protein 1 and fission protein 1, were determined in isolated perfused rat hearts subjected to I (30 min)/R (45 min). We found that P110 (1 µM) given for 10 min before ischemia and for 20 min post-reperfusion, significantly restored left ventricular developed pressure (LVDP) and the peak of the first derivative of left ventricular pressure (dP/dtmax) to 77 ± 9% and 58 ± 7% of baseline values at 45 min post-reperfusion, respectively, as compared with untreated control I/R hearts (both p<0.01, n=6). The LVDP and dP/dtmax of untreated control I/R hearts (n=8) only recovered to 34 ± 8% and 29 ± 4% of baseline values at 45 min post-reperfusion, respectively. P110 also significantly reduced infarct size to 20 ± 3% compared to 41 ± 4% in untreated control I/R hearts (p<0.01). The preliminary results suggest that inhibition of mitochondrial fission during MI/R improves post-reperfused contractile function and reduces infarct size. This study was supported by the Division of Research and the Department of Bio-Medical Sciences at Philadelphia College of Osteopathic Medicine.