The electrochemical activity determination of γ-glutamyl-transpeptidase is described using the electrogenic L-γ-glutamyl paraaminodiphenylamide substrate. In order to correlate the results with classical measurements involving chromogenic substrates, linear calibration curves for the amperometric detection of the released free amine, the kinetics of the catalysis and the enzyme level in one plasma sample have been compared to photometric experiments effectuated in parallel runs, in the same conditions, with L-y-glutamyl paranitroanilide as the substrate. Results show that this new technique applies successfully to such evaluations.