One of the major disadvantages of the use of 131I in the treatment of thyrotoxicosis is the development of hypothyroidism. Alternatively, 125I has been proposed for thyrotoxicosis therapy, and was thought to be preferable to 131I because of the short range of its emitted soft electrons. Several studies have shown 125I to be as effective as 131I in the treatment of thyrotoxicosis, and equally likely to produce hypothyroidism. This work compared the radiobiological effects of 131I and 125I given in doses to deliver the same amount of radiation to the rat thyroid gland. These effects were studied by in vivo determination of single-strand DNA breaks by alkaline sucrose gradient sedimentation using the DABA fluorescent technique to detect the DNA. Serum T4 and TSH concentrations and percentage T3 uptake were determined by RIA. The incidence of hypothyroidism following 131I and 125I therapy was found to be the same (10% in each group). The extent of DNA damage following 125I therapy was greater than the damage induced by a larger dose of 131I.