The electrochemical oxidation of reduced glutathione (GSH) catalyzed by 10-methylphenothiazine (MPT) at a glassy carbon electrode in 0.1 M (Et)(4)NClO4 ethanol + water (1:1 v/v) solution has been studied. Addition of MPT dramatically reduces the over-potential necessary for the oxidation of GSH and the catalytic current is affected by the concentration of the catalyst and the pl-l of the solution. The possible electrocatalytic reaction mechanism for the electro-oxidation of GSH in the presence of MPT was proposed. Oxidized glutathione is the oxidation product which has been verified by thin-layer chromatography and by melting-point and optical rotation measurements. A nontoxic, stable, and convenient method for the measurement of GSH in the concentration range 5.0 x 10(-6) to 1.76 X 10(-3) M has been developed.