A selective and sensitive method has been developed for the determination of sulphur amino acids by gas chromatography (GC). Sulphur amino acids were converted into their N(S)-isopropoxycarbonyl methyl ester derivatives and measured by GC with flame photometric detection using a DB-17 capillary column. The derivatives were sufficiently volatile and stable to give single symmetrical peaks. The detection limits of sulphur amino acids were ca. 0.5-1 pmol per injection, and the calibration curves were linear in the range 0.5-10 nmol for each sulphur amino acid. This method was successfully applied to small urine samples without prior clean-up, and sulphur amino acids in these samples could be analysed without any influence from coexisting substances. Overall recoveries of sulphur amino acids added to urine samples were 85-113%. The analytical results of free sulphur amino acid contents in urine samples of normal subjects are presented.