PET is a functional imaging technique suitable for studies of brain ammonia metabolism. Dynamic 13N-ammonia PET yields time-courses of radioactivity concentrations in brain (PET camera) and blood (samples). Ahl et al. (Hepatology 40:73–79, 2004) and Keiding et al. (Hepatology 43:42–50, 2006) analysed such data in patients with HE by a kinetic model accounting for transfer of 13N-ammonia across the blood–brain barrier (BBB) and intracellular formation of 13N-glutamine. Initial unidirectional 13N-ammonia transfer across BBB was characterized by the permeability-surface area product PSBBB (ml blood min−1 ml−1 tissue). There was a tendency to lower PSBBB values in patients with cirrhosis and HE than in patients with cirrhosis without HE and healthy controls but the differences were not statistically significant. Keiding et al. (Hepatology 43:42–50, 2006) also calculated PSmet (ml blood min−1 ml−1 tissue) as a measure of the combined transfer of 13N-ammonia across BBB and subsequent intracellular metabolism of 13N-ammonia; neither did this PS-value show significant difference between the groups of subjects. Net flux of ammonia from blood into intracellular metabolites was linearly correlated to arterial ammonia. In conclusion, basic brain ammonia kinetics was not changed significantly in patients with cirrhosis +/- HE compared to healthy controls. Blood ammonia seems to be the more important factor for increased brain ammonia uptake in HE.