Abstract: The complex ~(99)Te~m-ethyl cysteinate dimer (~(99)Tc~m-ECD) which is a valuable brain imaging agent is prepared through the ligand exchange reaction between ~(99)Tc~m-glucoheptonate (GH) and ECD. In order to explore the exchange reaction between the ~(99)Tc~m-GH and N_2S_2 type ligands as a common methodfor the preparation of ~(99)Tc~m-N_2S_2 complexes, the ligand exchange reaction between ~(99)Tc~m-GH and ECD was studies at different pH values, reaction time, ECD and Na-GH concentrations. The exchange ratio was determined by paper chromatography, and the reaction rate constants were calculated at different ECD concentration (pH=6) and different pH values. The results obtained show that the ligand exchange between ~(99)Tc~m-GH and ECD is a second order reaction and the rate constant of the ligand exchange reaction depends on pH values.