Abstract: For clarifying some possible mechanism of cadmium toxicity, the effect of increasing amounts of Cd2+ ion on peroxidase activity was investigated in vitro in serum of cow. The H2O2-mediated oxidation of o-dianisidine was used to assess the peroxidase activity. Results show that after preincubation of serum with 0.25-100 mM Cd2+ concentration for 5 min, peroxidase activity was inhibited compared to the control and decreased rapidly with increasing metal concentrations. The enzyme was completely inhibited after 5 min preincubation in 100 mM Cd2+. When the preincubation of serum and Cd2+ was prolonged to 0.5, 1 and 24 h, the enzymatic activity decreased more rapidly with increasing metal concentration and the enzyme was completely inhibited at lower metal concentrations (at 50 mM Cd2+ after 30 min preincubation, at 30 mM Cd2+ after 60 min preincubation and at 5 mM Cd2+ after 24 h). By considering of long biological half-life of cadmium in body of animals, it suggested that the damage caused by exposure to heavy metals is often not only dose-dependent, but also time-dependent. Even though detoxifying enzymes may not show any effect after brief exposure to low concentrations of heavy metals, prolonged incubation will affect the enzymatic activity, leading eventually to complete inactivation.