A new HPLC method for the determination of 5-S-cysteinylcatechols has been developed. The alumina adsorbed fraction of the supernatant of brain homogenate was injected onto a reversed-phase column and a citrate-phosphate buffer containing 1-nonyl sulphate was used as mobile phase (pH 2.1). Two dual-series working electrodes of a thin-layer cell were operating together, joined by a special coupler. The assay allows determination of the 5-S-cysteinylcatechols in the striatum, limbic system and mesencephalon of one guinea pig. Recoveries of the three 5-S-cysteinylcatechols were 59-76%, whereas the limit of quantitation was 0.04-0.10 pmol. The coefficient of variation was less than 0.76-1.10% and linearity was found up to a concentration of 500 pmol. By adding ascorbic acid to the samples, artifacts resulting in HPLC peaks were either reduced in size or deleted.