A selective and sensitive high performance liquid chromatographic (HPLC) method has been developed for the determination of reduced glutathione (GSH) in biological samples (rat liver, spleen and plasma). The method involved a prechromatographic thiol derivatization with methyl 4-(6-methoxynaphthalen-2-yl)-4-oxo-2-butenoate; the reaction was rapid (5 min) under mild conditions (pH 7.5 and ambient temperature) and selective for the sulphydryl group. The thiol adducts were separated on a reversed-phase C-18 column using acetonitrile: 0.05 M triethylammonium (TEA) phosphate (pH 4) solution 32:68 (v/v) as the mobile phase. Fluorescence detection (lambda em = 450 nm; lambda exc = 310 nm) was used and the detection limit (S/N = 3) was about 0.5 pmole of the injected GSH adduct. The method was also applied to the determination of total glutathione in rat plasma after a preliminary reduction with dithiothreitol.