Using N-(1-pyrene)maleimide as a reagent for thiol compounds and high-performance liquid chromatography with fluorometric detection, we have identified cysteinylglycine as an endogenous compound in dithiothreitol-reduced urine. In a quantitative method developed for cysteinylglycine, reduction of urinary disulphides was effected by dithiothreitol at pH 6. The pH was then brought to 1.5 and excess dithiothreitol together with acid thiols was extracted with water-saturated ethyl acetate. After derivatization the concentration was determined by reversed-phase liquid chromatography. Precision of the method (C.V. = 6.5%) and analytical recovery (86 +/- 6.4%) were satisfactory. The urinary excretion of cysteinylglycine was 7.4 +/- 2.3 mumol/l (mean +/- S.D.) in eight healthy subjects.