In case of occuring the atomic energy accidents the proper medical treatments should be necessary. As the aim of the basic data for protective actions, the present studies were carried out to evaluate the decontamination of radioiodine by the administration of the antithyroid drugs (KI, NaI) and isotonic saline. Some recommended methods of decorporating radioiodine were investigated using 450, NIH-GP mice, each injected intraperitoneally with $1{mu}Ci$ of $NaI^{131}$ as the internal contamination and treated with 2mg/0.2ml-saline of NaI and 2.6mg/0.2ml-saline of KI as the antithyroid drugs. Accordingly, effects of antithyroid drugs for internal contamination were: 1. Administration of NaI and KI caused to rapidly excrete internal radioiodine as the antithyroid drugs and decrease in whole body retention was reduced than in the saline group. 2. After internal contamination NaI and KI were to be administered for radioprotective effects as quickly as possible. 3. Decrease in body-retention made temporary shifts with enough fluids (water), however, as far as radioprotective effects is concerned, saline was not more significant than in the other group (NaI and KI). 4. Regarding to thyroid protective effects NaI, KI and saline were significant in effectively order.