The 5-S-cysteinyl derivatives of dopamine, dopa (3,4-dihydroxyphenylalanine) and dopac (3,4-dihydroxyphenylacetic acid) were synthesized and used as reference compounds in high performance liquid chromatography analyses of extracts from various brain regions of eight mammalian species. All three metabolites were detected in the brains of all the species studied. The regional distribution of the metabolites was similar to that of dopamine; the metabolite concentrations ranged from less than 0.1 percent to more than 1 percent of the dopamine level, the highest ratios generally being found in substantia nigra. It is suggested that the 5-S-cysteinyl catechol metabolites have been formed after autoxidation of catechols to quinones and subsequent coupling to glutathione. The adduct thus formed is finally split by peptidases to yield the 5-S-cysteinyl derivatives.