The differences in the change of the uptake rate of radioactive iodine ($^{131}I$) in the thyroid gland after $Tapazole^{(R)}$ administration before $^{131}I$ treatment were analysed in 137 patients who were diagnosed as diffuse toxic goiter in the Radioisotope Clinic and Laboratory, Seoul National University Hospital since Jan. 1967 to July, 1969. The uptake rate of the therapeutic dose of $^{131}I$ was changed diffusely compared with that of the trace dose in the patients who had no $Tapazole^{(R)}$ administration before $^{131}I$ treatment. In those patients who had $Tapazole^{(R)}$ more than 20 mg/day for more than one week before $^{131}I$ treatment, the uptake rate was decreased significantly. When the patients discontinued the administration of $Tapazole^{(R)}$ 7 days prior to $^{131}I$ treatment, the uptake rate was increased in all cases.