It is well known that γ-aminobutyric acid (GABA) is an important inhibitory neurotransmitter in the mammalian nervous system. GABA levels in cerebrospinal fluid (CSF) are considered to be an indicator of GABAergic neuronal functions in the central nervous system. A decrease of the GABA level in CSF has been reported in several neurological disorders, as assayed by ionexchange-fluorimetric and radioreceptor assay methods. Automatic amino acid analysers have been used to investigate whether the amounts of amino acids in human CSF are altered in neurological and mental diseases. Owing to the low concentration of GABA in CSF, however, the increases of aspartic acid and glutamic acid, excitatory neurotransmitters, in the CSF from epileptic patients were reported. In recent years, the fluorogenic reactions of amino acids with o-phthalaldehyde (OPA) and thiol have been exploited in reversed-phase high-performance liquid chromatography (HPLC). Several HPLC systems have been used to measure concentrations of amino acids in CSF. However, the concentration of GABA in CSF was reported in only some papers, because it is less than 10% of that of the other amino acids in CSF. This paper describes a rapid isocratic HPLC method with fluorimetric detection for the determination of the OPA derivative of GABA in human CSF.