A new flow injection potentiometric (FIP) method, rapid, reproducible and simple to apply, has been developed for the in vitro evaluation of antioxidative capacity of aqueous plant extracts. This method is based on the transient negative signal measurements with a flow-type platinum electrode detector due to the composition change of a [Fe(CN)6]3-/[Fe(CN)6]4- redox-reagent solution. The variables affecting the signal height such as composition and concentration of redox-reagent, injected sample volume, flow rates of carrier and redox-reagent solution streams were studied in details and the conditions were optimized. For the compounds under study, a linear relationship was stated between the potentiometric signal height and the logarithm of antioxidant concentration. It was stated that a wide antioxidant activity range from 1 microM to 10 mM could be determined by the changing concentrations of the hexacyanoferrate(III) from 5 to 0.01 x 10(-4). The present FIP method was applied to quantify relative antioxidant activity (RAA index) of the representative water-soluble antioxidants (ascorbic acid, pyrocatechol, pyrogallol, caffeic acid, chlorogenic acid, gallic acid, tannic acid, uric acid, L-cysteine, trolox). The high sampling rate (100 h(-1)) and a satisfactory reproducibility (R.S.D.=0.7-1.8%, n=5, 0.1 mM each compound) were obtained. The method was also applied to estimate total antioxidant activity (TAA) of real samples (green and black tea infusions, herbal infusions and fresh fruit extracts) and the results were compared with those achieved using well-known in vitro testing methods.