To clarify the nature of nitrogen metabolism between branched chain amino acid (BCAA) and glutamine (Gln) in liver failure, we measured arterial plasma concentrations of Gln and 15N uptake to amino-N and amide-N of Gln in normal and D-galactosamine-induced fulminant hepatic failure (FHF) rats after 15N-leucine (Leu) injection. Fifteen, 30 and 60 min after Leu injection, the arterial plasma concentrations of Gln were significantly higher in FHF rats than in controls. The concentrations of amino-15N Gln were also significantly higher in FHF rats than in controls at 5, 15, 30 and 60 min after injection. The concentrations of amide-15N Gln did not significantly differ between FHF and controls at 5, 15 and 30 min. However, at 60 min, the concentration was significantly higher in the FHF rats. The higher uptake of 15N to amino-N of Gln in FHF rats suggests the presence of an enhanced ability to synthesize Gln from Leu in FHF rats. The higher uptake of 15N to amide-N of Gln in FHF rats at 60 min after injection suggests that excessive administration of BCAA to patients with severely impaired urea-cycle capacity suffering with hepatic failure may lead to greater levels of hyperammonemia.