Cyclic voltammetry, differential-pulse voltammetry (DPV), amperometry and flow injection with electrochemical detection were used to investigate the electrochemical behaviour of reduced glutathione (GSH) at a carbon paste electrode and also at a carbon paste electrode doped with cobalt phthalocyanine (CPC). The modified electrode was found to reduce greatly the overpotential necessary for the oxidation of GSH at the electrode surface; the process was dependent on the ionic strength and pH of the supporting electrolyte. The oxidation of GSH in the presence of CPC was found to occur at two potentials, 0.27 and 0.75 V versus a saturated calomel electrode. Using DPV the first wave was found to give optimum sensitivity for the measurement of GSH concentations in the range 2.5 × 10^–6–6.25 × 10^–5M; whereas the second was preferential for GSH concentrations of 1.25 × 10^–4–2.5 × 10^–3M. Electrode deterioration rendered the other techniques unsuitable for routine quantitative analyses. Similar investigations were also carried out on a carbon-epoxy resin composite electrode containing CPC; the various techniques were used to optimise the electrode response for the determination of GSH in biological fluids. This electrode was found to be superior to the graphite paste electrode as GSH could be determined in flowing or stirred solutions. The modified composite electrode was stable and could be used over a period of at least 10 h. The limits of detection were 10 ng ml^–1 and 450 pg injected on column for amperometry and high-performance liquid chromatography with electrochemical detection (LCEC), respectively. Preliminary studies using LCEC have shown that the method is suitable for the determination of circulating levels of GSH in human plasma.