This experiment was carried out to investigate the function of thyroid during the molting period in the hen by radioiodine techniques and histological observations of the thyroid gland. It was also investigated the change in the radioactivity of the blood, excreta and egg after a single intramuscular injection of I^(131). Five laying hens and five molting hens were injected 2 μc/100g. body weight with a single intramuscular injection of carrier frees I^(131) to measure the rate of thyroid I^(131) uptake, the release rate, Protein Bound Iodine^(131) (PBI^(131)) and PBI^(131) conversion ratio. The results of this experiment were as follows: 1. The maximum ratio of thyroid I^(131) uptake found in 12 hrs. after I^(131) administration, of laying(27.92%), molting(42.01%) was significantly (P$lt;0.05) different between laying and molting hens. At the end of observation (i.e. after 132 hours following the peak of thyroid I^(131) uptake percentage) the rate of thyroid I^(131) release for molting hens (0.2234%/hr.) was significantly (P$lt;0.05) higher than that for laying hens (0.0834%/hr.). 2. The maximum activity of PBI^(131) occurred 24 hrs. after I^(131) administration. Molting hens are always higher than that for laying hens up to 96 hrs. and there was significantly (P$lt;0.05) different between laying and molting hens after 48 hrs. However, PBI^(131) conversion ratio was not significantly different. 3. The size of thyroid follicle of ;molting hens (73.13μ) was significantly smaller than that of laying hens (108.71μ). However, the height of follicle epithelial cell for molting hens (5.14μ) was significantly (P$lt;0.05) greater than that for laying hens (3.56μ). It was observed that the follicles in molting hens lost some of colloidal substances, while those in laying hens were full of colloidal substances. 4. The concentration of I^(131) in blood was found to be highest in 12 hrs. following injection of radioactive iodine and was decreased rapidly in 24 hrs. Thereafter a continuous decrease in concentration was observed. 5. The excretion ratio of I^(131) after 24 hrs. following administration was found to be 58.40% for laying hens and 46.89% for molting hens. The laying hens excreted significantly (P$lt;0.05) more I^(131) than molting hens did. The ratio was sharply decreased after 48 hrs. following injection: 6. The change of radioiodine contents in the laid eggs was peculiar. In the shell of egg little radioactivity was detected. I^(131) in the egg white and egg shell decreased in the same way as in the blood. Although no radioactivity was detected from the egg yolk after 24 hrs. following injection of I^(131), the peak of activity containing 3.62% of injected dose was observed during 4 to 6 days of experiment.