*Purpose: We aimed to characterize the performance of old vs. young liver grafts in a standardized rat model of normothermic ex vivo liver machine perfusion (NEVLP), as liver grafts are frequently declined due to high donor age or age mismatch with the recipient.

*Methods: Livers from Sprague Dawley rats aged 3 months or 12 months were harvested and either perfused for 6 hours using a rat dual vessel NEVLP system or collected as a reference group of non-perfused livers for each age (n=6/group). Tissue and perfusate samples were used for histological and biochemical analyses.

*Results: Livers of older animals were significantly heavier (liver weight: 20.2 g vs. 17.2 g; p=0.002). Arterial and portal venous pressure, bile production as well as perfusate electrolytes did not differ between groups and remained in between the respective reference ranges throughout the perfusion. All livers cleared lactate during perfusion from a peak of 6.9 mmol/l to 1.6 mmol/l and continued to produce bile after 6 hours of perfusion (614 mg/h IQR 228mg). Bile pH was within reference range (median 8.17 IQR 0.11). Peak transaminase levels in 12-month-old animals were slightly higher (ALT: 200 U/l vs. 272 U/l; p=0.2). Urea levels increased throughout the perfusion and were significantly higher in older animals (12.7 mmol/l vs. 16.3 mmol/l; p=0.04). H&E staining showed vital liver tissue and no significant differences between age groups. Sirius red staining showed slightly higher fibrosis in 12-month-old animals.

*Conclusions: Liver grafts from 3- and 12-month-old Sprague Dawley rats performed equally well on NEVLP. Future studies will address the outcome after transplantation of old vs. young rat livers with static cold storage vs. NEVLP.

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