A square wave voltammetric method was developed for the detection of glutathione reductase (GR) activity. The method is based upon the direct determination of glutathione (GSH) produced by nicotinamide adenine dinucleotide phosphate (NADPH)-dependent reduction of glutathione disulfide (GSSG). Enzyme activity was represented by an increase in steady-state reduction current of GSH and this current was monitored voltammetrically. At the optimized working condition, reduction potential of GSH was found at -0.44 V with a hanging mercury drop electrode versus an Ag/AgCl electrode. The reduction current was directly proportional to GSH concentration in the range 2.63-800 mu M with a lower detection limit of 0.79 mu M and lower quantification limit of 2.63 mu M. Inhibitory activities of four antimony(III) compounds were determined by this method, and obtained IC50 values were compared with previous data. In addition, an electrochemical study of these compounds showed that their reductions have an EC mechanism; the current is diffusion controlled, and E-p/2 values are proportional to inhibitor activity.