Contact radioautographs were made from sections of freeze-dried thyroids of 131I injected rats, mice and guinea-pigs in which organic binding of iodine had been prevented with propylthiouracil. Some of the animals were previously untreated, others had received propylthiouracil in the drinking water for periods of 10 or 60 days. The radioautographs were made with dry thin stretched stripping film. The iodide was found to be distributed right through the follicles and appeared in higher concentration in the colloid than in the cells.