Effects of Co+2, Zn+2, Ca+2, Fe+2, Mn+2, Cr+3, Sn+2 and Mg+2 exposure of rainbow trout liver on the enzymatic activity of glutathione reductase (GR) were investigated. GR was purified from rainbow trout liver 1419-fold with a yield of 38.41% and a specific activity of 22.846 U/mg protein. Following enzyme isolation, inhibitory effects of Co+2, Zn+2, Ca+2, Fe+2, Mn+2, Cr+3 and Sn+2 were analysed. IC50 values of the metal ions ranged from 42.2 to 657 mu M, and the K-i constants ranged from 24.2 to 567 mu. Co+2, Zn+2, Fe+2 and Cr+3 exhibited competitive inhibition whereas others inhibited the enzyme in non-competitive manner. Cobalt was the most powerful inhibitor among others having the lowest IC50 and K-i values. Magnesium exhibited activatory effect on the enzyme.