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. IC(50) values of the metal ions ranged from 42.2 to 657 µM, and the K(i) constants ranged from 24.2 to 567µ. 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 IC(50) and K(i) values. Magnesium exhibited activatory effect on the enzyme.