Introduction: Hepatic encephalopathy (HE) is a neuropsychiatric disorder, a major complication of chronic liver disease (CLD). Hyperammonemia is central in the pathogenesis of HE as ammonia crosses the blood-brain barrier (BBB) causing toxicity. Glutamine synthetase (GS), an enzyme which removes ammonia during the amidation of glutamate to glutamine, is expressed in liver, muscle and brain. Since liver is responsible for regulating blood ammonia levels, extra-hepatic GS plays an important compensatory role during CLD. However, GS expression in endothelial cells (EC) of the BBB has never been explored. Methods: GS protein and activity was assessed in 1) rat brain microvascular EC (+/- ammonia exposure and plasma from rats with CLD) and 2) isolated cerebral microvessels (CMV) from naïve rats. Results: GS was co-localized with EC in brain of naïve rats. GS protein and activity was detected in CMV and in vitro, but with lower levels compared to brain (p<0.05). EC exposed to ammonia resulted in increased GS activity (p<0.05). However, ECs exposed to plasma from CLD rats resulted in lower GS activity and protein expression compared to controls (p<0.05). Conclusion: We demonstrate for the first time the presence of GS in EC in both in vitro and in vivo. Stimulated by ammonia, GS is however reduced following exposure to plasma from hyperammonemic CLD rats. This suggests other systemic factors such as oxidative stress (present in CLD; Bosoi et al., Free Radic Biol Med, 2012) could hinder GS activity. We speculate a downregulation of GS in the BBB during CLD leads to rapid entry of ammonia into the brain and the development of HE. Hence, upregulating GS in the BBB could become a new therapeutic target for HE.