(Introduction and purpose:) under bar Lithium is an element that has been widely used in therapy of mental diseases. In different studies it has been shown that it interacts with enzymes and alters the function of proteins in organisms. On the other hand fibrinogen is one of the plasma proteins that can interact with euglobulin which is the tissue plasminogen activator indicator. Present study was planned to assess the effect of lithium on alteration of function of proteins that play a role in the fibrinolytic system in blood. Materials and methods: Control group of rats was fed a normal diet whereas pharmacological and toxic groups received 0.4 mmolLi/kg/day and 6.6 mmolLi/ kg/day for 2 weeks, respectively. Blood samples were drawn from animals and fibrinolytic activity was measured by Coppley method, which is based on the euglobulin lysis time (ELT). Fibrinogen concentration determinations were per-formed by Ratnoff and Menzie method. Results and conclusion: After evaluation of fibrinolytic activity rates that belong to both groups, it has been observed that lithium altered the activities of proteins that are responsible for fibrinolysis. Fibrinogen concentrations of both experimental groups were found to be lower than that of control group values (p < 0,05 in pharmacological group, p < 0.001 in the toxic group). On the other hand ELT values in the toxic dose lithium administered group was significantly shortened than control group values (p < 0.001).