AbstractRats were given intravenous15NH4+infusion at a rate of 2.2 or 5.5 mmol/h/kg body wt to induce hyperammonemia, as animal models of hepatic encephalopathy. Its effect on cerebral amino acid metabolism was studiedin vivoby15N NMR spectroscopy at 20.27 MHz for15N. Cerebral [γ‐15N]glutamine (present at a tissue concentration of 4–9 μmol/g) and [α‐15N]glutamate/glutamine (6 μmol/g) were clearly observed in living rats within 9‐18 min. In portacaval‐shunted rats, final cerebral [γ‐15N]glutamine concentrations were higher than those in controls after the same infusion period, presumably because decreased15NH 4+removal in the liver led to increased15NH3diffusion into the astrocytes. In control rats, cerebral [γ‐15N]glutamine pool increased at a rate of 1.7 μmol/h/g when blood ammonia concentration was 0.8 mM.15N enrichment in γ‐15N was 71%. From these observations,in vivoactivity of glutamine synthetase in rat brain was estimated to be 3.5 μmol/h/g. Comparison with reported optimumin vitroactivity suggests thatin situconcentrations of some substrates and cofactors limit the activity of g