Grafts from non-heart-beating donors are thought to be best preserved by hypothermic machine perfusion (HMP). Controversy exists concerning the role of oxygenation during HMP. In this study, we wanted to evaluate the relative role of oxygenation for graft integrity during and after HMP. Cardiac arrest was induced in male Wistar rats (250-300 g) by phrenotomy. Thirty minutes later, livers were flushed via the portal vein and subjected to 18 h of HMP at 5 ml/min at 4 degrees C. During HMP, the preservation solution was equilibrated with 100% oxygen (HMP100), with air (HMP20) or not oxygenated at all (HMP0). Graft integrity was assessed thereafter upon warm reperfusion in vitro. During preservation, oxygenation of the perfusate reduced alanine aminotransferase release by 50% compared with HMP0. HMP100 resulted in reduced oxygen free radical-mediated lipid peroxidation upon warm reperfusion compared with both HMP20 and HMP0. One hundred per cent oxygenation during HMP also significantly enhanced the activation of AMPK salvage pathway, and upstream activation of protein kinase A when compared with HMP0. Enzyme release during reperfusion was reduced by approximately 40% (HMP20) or approximately 70% (HMP100) after oxygenation compared with HMP0. Functional recovery (bile production) was only enhanced by HMP100 (approximately twofold increase vs. HMP20 and HMP0, P < 0.05). Efficiency of HMP might be markedly increased by additional aeration of the perfusate, most successfully by equilibration with 100% oxygen.