This paper describes a high-performance capillary electrophoretic (HPCE) method which allows a quick, simultaneous and quantitative determination of reduced (GSH) and oxidized (GSSG) glutathione in mammalian red blood cells using a Supelco-bonded hydrophilic phase capillary CElect-P150. The extraction procedure of GSH and GSSG from erythrocytes using Microcon-10 membranes is very simple and allows a correct evaluation of these compounds present in the red blood cells. Furthermore, the HPCE method does not require removal of the excess N-ethylmaleimide used to block the glutathione in its reduced state, making the simultaneous evaluation of GSH and GSSG possible in a very short time (ca. 4 min), with a sensitivity at femtomole level.