Aims: Acute-on-chronic liver failure (ACLF), an acute deterioration of liver function during compensated chronic liver disease, is associated with the development of hepatic encephalopathy (HE). Brain edema is frequently observed in patients with HE induced by either acute liver failure (ALF) or chronic liver disease. We recently demonstrated in a rat model of cirrhosis that hyperammonemia and systemic oxidative stress synergistically lead to brain edema. Therefore, the aim of this study was to develop a rat model of ACLF by inducing an acute liver insult, superimposed onto a chronic hyperammonemia background, and to investigate the role of oxidative stress and ammonia in relation to brain edema. Methods: ACLF was induced in male Sprague-Dawley rats by portacaval shunt (PCA), followed 4 weeks later by hepatic artery ligation (HAL). Liver devascularisation by concomitant PCA and HAL was used to induce ALF. SHAM and PCA (chronic hyperammonemic) rats, sacrificed 4 weeks after surgery, were included as controls. Liver status (routine biochemistry and histopathology), blood ammonia and brain edema (specific gravimetric technique) were assessed. Oxidative stress was evaluated by plasmatic and cerebral levels of reactive oxygen species (DCFDA fluorescence), glutathione (DTNB method), and activities of xanthine oxidase and catalase (Amplex Red method). Results : Coma developed in ALF rats 8h after HAL; it was significantly delayed in ACLF rats, where it occurred after 16h. Liver necrosis markers AST and ALT did not differ between pair-killed ACLF and ALF rats; however, liver histopathology showed more severe necrosis in ACLF than in ALF rats. Brain water content increased in ALF rats and was significantly attenuated in ACLF rats: 80.04±0.13% p<0.01vs ALF (81.39±0.15%), n.s. vs SHAM. The increase in arterial ammonia, as seen in ALF rats, was prevented in ACLF rats: 0.35±0.07 mM, p<0.001vs ALF (1.34±0.09 mM), p<0.01 vs SHAM. Oxidative stress was present in the blood of both ACLF and ALF rats, while signs of oxidative stress in the brain were present only in the ALF rats. Conclusions: The alterations observed in ALF are attenuated in ACLF. In spite of a more severe hepatic necrosis in the ACLF rats, the onset of coma was delayed, and brain edema, ammonia levels and oxidative stress were reduced in comparison to the ALF rats. In conclusion, severe HE is alleviated by compensatory mechanisms comprising oxidative stress, which are developed during chronic hyperammonemia prior to an acute liver deterioration. A better understanding of these mechanisms may help define the pathogenesis of ACLF.