Glutathione reductase (GR; E.C 184.108.40.206) catalyzes the reduction of oxidized glutathione to reduced glutathione by the nicotinamide adenine dinucleotide phosphate (NADPH) dependent mechanism. GR participates in the protection of the cell from the toxic effects of free radicals. In this study, we have investigated in vitro effects of isoorientin (C-glycosyl flavonoid), verbascoside, and forsythoside B (phenylethanoids) on purified bovine kidney cortex GR. These compounds present in many plant species that have been used as medicinal plants and have an interesting spectrum of biological activity. We have demonstrated that these compounds inhibit bovine kidney cortex GR in a concentration-dependent manner. Kinetic characterization of the inhibition is also done. Isoorientin is a noncompetitive inhibitor with respect to both glutathione disulfide (GSSG) (KiGSSG 0.023 ± 0.001 mM) and NADPH (KiNADPH 0.032 ± 0.001 mM). Forsythoside B acts as an uncompetitive inhibitor with respect to GSSG (KiGSSG 0.20 ± 0.014 mM) and NADPH (KiNADPH 0.267 ± 0.028 mM). Verbascoside acts as an uncompetitive inhibitor with respect to both GSSG (KiGSSG 0.242 ± 0.024 mM) and NADPH (KiNADPH 0.175 ± 0.016 mM). Inhibition of GR causes accumulation of reactive oxygen species and depletion of the glutathione pool. Inhibition of this enzyme may be significant in drug resistance.