4-(N,N-Dimethylaminosulphonyl)-7-fluoro-2,1,3-benzoxadiazole (DBD-F) was synthesised for use as a more reactive, thiol-specific fluorogenic reagent than 4-(aminosulphonyl)-7-fluoro-2,1,3-benzoxadiazole (ABD-F). The former had negligible fluorescence whereas its thiol derivatives fluoresced intensely at about 510 nm (excitation occurred at about 380 nm). The DBD-F reacted quantitatively with thiols after 10 min at 50 degrees C and pH 8.0 and the reaction rates were several times higher than those with ABD-F; it is suggested that the electron withdrawing effect of the dimethylsulphonamide group (SO2NMe2) is larger than that of the sulphonamide group (SO2NH2). No reaction occurred with alanine, proline, cystine or cysteic acid under the same conditions. The fluorescence intensities of the derivatives were found to be higher in neutral and acidic media than in alkaline solutions. The thiol derivatives of DBD-F were separated by high-performance liquid chromatography and detected fluorimetrically, the detection limits being 0.92, 0.16, 0.13, 0.16 and 0.32 pmol for cysteine, glutathione, homocysteine, N-acetylcysteine and alpha-mercaptopropionylglycine, respectively. The method was applied to the determination of thiols in rat tissues.