Two mechanisms may account for brain edema in fulminant hepatic failure: the osmotic effects of brain glutamine, a product of ammonia detoxification, and a change of cerebral blood flow (CBF). We have shown brain edema, a marked increase in brain glutamine, and a selective rise in CBF in rats after portacaval anastomosis receiving an ammonia infusion. In this study, we inhibited the activity of glutamine synthetase with methionine-sulfoximine (MSO) and examined ammonia levels, brain water and CBF. Four groups received either a continuous ammonium acetate or control infusion; half of the animals had been pretreated with MSO or vehicle. The ammonia group exhibited brain edema (79.97 ± 0.04 vs. 81.11 ± 0.13% water), an increase in cerebrospinal fluid (CSF) glutamine (1.29 ± 0.21 vs. 2.84 ± 0.39 mmol/L) and CBF (63 ± 11 vs. 266 ± 45 mL/min/100 g brain). When MSO was added to the ammonia infusion, ammonia levels rose further (928 ± 51 vs. 1,293 ± 145 mmol/L, P < .05) but CSF glutamine decreased (2.84 ± 0.39 vs. 1.61 ± 0.2 mmol/L, P < .01). Brain edema (80.48 ± 0.11%) and cerebral hyperemia (140 ± 25 mL/min/100 g brain) were significantly ameliorated in the ammonia plus MSO group. Brain output of circulating nitric oxide (NOx) was increased in the ammonia-infused group but normalized in the ammonia plus MSO group. In this model, the rise of CBF reflects intracranial events that occur after glutamine synthesis. Activation of nitric oxide synthase in the brain could account for these findings.