The differences in the change of the uptake rate of radioactive iodine (131I) in the thyroid gland after Tapazoleⓡ administraion before 131I treatment were analyzed 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 131I was changed diffusely compared with that of the trace dose in the patients who had no Tapazoleⓡ administraion before 131I treatment. In those patients who had Tapazoleⓡ more than 20mg/day for more than one week before 131I treatment, the uptake rate was decreased significantly. When the patients discontinued the administration of Tapazoleⓡ 7 days prior to 131I treatment, the uptake rate was increased in all cases.