Brain edema sufficient to cause intracranial hypertension and brain herniation remains a major cause of mortality in acute liver failure (ALF). Studies in experimental animal models of ALF suggest a role for ammonia in the pathogenesis of both encephalopathy and brain edema in this condition. As part of a series of studies to evaluate the therapeutic efficacy of ammonia-lowering agents, groups of rats with ALF caused by hepatic devascularization were treated with L-ornithine-L-aspartate (OA), an agent shown previously to be effective in reducing blood ammonia concentrations in both experimental and human chronic liver failure. Treatment of rats in ALF with infusions of OA (0.33 g/kg/h, intravenously) resulted in normalization of plasma ammonia concentrations and in a significant delay in onset of severe encephalopathy. More importantly, brain water content was significantly reduced in OA-treated rats with ALF. These protective effects of OA were accompanied by increased plasma concentrations of several amino acids including glutamate, ␥-aminobutyric acid (GABA), taurine, and alanine, as well as the branched-chain amino acids, leucine, isoleucine, and valine. Increased availability of glutamate following OA treatment provides the substrate for the major ammonia-removal mechanism (glutamine synthetase). Plasma (but not cerebrospinal fluid) glutamine concentrations were increased 2-fold (P F .02) in OAtreated rats, consistent with increased muscle glutamine synthesis. Direct measurement of glutamine synthetase activities revealed a 2-fold increase following OA treatment. These findings demonstrate a significant ammonia-lowering effect of OA together with a protective effect on the development of encephalopathy and brain edema in this model of ALF. (HEPATOLOGY 1999;30:636-640.)Brain swelling culminating in increased intracranial pressure and subsequent brain herniation remains the major cause of death in acute liver failure (ALF). Although the pathogenesis of brain edema in ALF has not been fully elucidated, there is a growing body of evidence to suggest that ammonia (either directly or indirectly) plays a predominant role. In experimental animal models of ALF resulting from hepatectomy, 1 hepatic devascularization, 2,3 or toxic liver injury, 4 brain edema is a consistent finding, and brain ammonia frequently reaches millimolar concentrations. Exposure of various brain preparations to millimolar concentrations of ammonia in vitro results in significant cell swelling. 5,6 Furthermore, precipitous increases in blood ammonia concentrations are associated with brain edema in conditions such as Reye' s syndrome 7 and urea cycle enzymopathies. 8 L-Ornithine-L-aspartate (OA) has been proven to be effective in lowering blood ammonia concentrations in both experimental and human chronic liver failure. 9-11 A recent randomized, clinical trial of OA treatment revealed significant reductions in blood ammonia in cirrhotic patients concomitant with a significant improvement of neuropsychiatric symptoms. 10 In view of these findin...