The aim was to automate the assay of plasma and erythrocyte glutathione peroxidase (GSH-Px) activity using the Hitachi 911 (Boehringer Mannheim). There is a need for a reliable and rapid technique for the determination of GSH-Px activity to evaluate 'the oxidative stress'. The technique used was that proposed by the Randox laboratories (Randox Laboratories Ltd, Ransel test, reference: RS-505). The Hitachi 911 was programmed by adapting the manual Randox technique. Repeatability, reproducibility and linearity were satisfactory, and usual values for plasma and erythrocyte GSH-Px activity were determined. Automation of the Ransel test for determination of GSH-Px activity was very useful for serial assays. In addition, the technique is practical and has good interlaboratory reproducibility.