Abstract: The electrocatalytic oxidation of glutathione (reduced form GSH) at 10-methylphenothiazine (MPT) modified carbon paste electrode (MPT/CPE) was investigated by cyclic voltammetry (CV). Although GSH itself showed a very poor electrochemical response at carbon paste electrode (CPE), the response could be greatly enhanced by using MPT/CPE, which enables a sensitive electrochemical determination of the substrate GSH. The reaction rate constant for catalytic oxidation was evaluated as (5.44±0.03)×10² (mol·L^-1)^-1·s^-1 by using chronoamperometry (CA). The catalytic oxidation peak current of GSH versus its concentration had a good linear relationship in the concentration range of 5.0×10^-6-2.0×10^-3 mol·L^-1 with the correlation coefficient of 0.999 0, and the detection limit of 1.0×10^-6 mol·L^-1 by linear sweep voltammetry (LSV). The method can be applied for the determination of GSH in injection samples with the satisfactory results.