The effects of halothane on brain amino acid contents were investigated in rat by using a gas chromatography-mass spectrometry system. The contents of glycine and glutamate did not show any consistent changes in 1% or 2% halothane, but increased in 3% or 4% halothane. The levels of gamma-aminobutyric acid were not altered in any concentrations of halothane. The levels of aspartate increased in a dose-dependent manner up to 3% halothane, but returned to the control level in 4% halothane. These findings suggest that halothane brings the alteration of brain amino acid contents by affecting their synthesis and catabolism.