A selective and sensitive method for the determination of total homocysteine (Hcy) and related aminothiols, such as cysteine (Cys) and cysteinylglycine (CysGly), in plasma samples by gas chromatography (GC) has been developed. After reduction of the sample with sodium borohydride, the liberated Hcy and other aminothiols were converted to their N,S-diisopropoxycarbonyl methyl ester derivatives and measured by GC with flame photometric detection using a DB-17 capillary column. The calibration curves were linear over the range 0.5-10 nmol for Hcy and CysGly, and over the range 5-100 nmol for Cys, and the correlation coefficients were above 0.996. Using this method, total plasma Hcy, Cys and CysGly could be directly analysed without prior clean-up of the sample and without any interference from coexisting substances. Overall recoveries of Hcy and other aminothiols added to plasma samples were 95-106%. Analytical results for the determination of total plasma Hcy, Cys and CysGly from normal subjects are presented.