The modifying effects of 1,000 rads x-radiation on the goitrogenic response of the rat thyroid to methylthiouracil administration were studied at intervals up to 1 year after irradiation. Compared to the normal goitrogenic response, 1,000 rads x-radiation caused no cell death but resulted in a marked diminution in the 10 day goitrogenic response observed immediately after irradiation. Over the following 12 months, there was still no evidence of cell death nor was there any further reduction in the over-all reproductive capacity of the thyroid follicular cell population compared to that observed immediately after irradiation. It is, therefore, concluded that a "radiation partial thyroidectomy" which requires the combination of significant cell death along with significant reproductive survival is intrinsically impossible in the rat using homogeneous organ irradiation. As 1,000 rads x-radiation does not produce a significant number of nonfunctioning viable follicular cell mutants, a "functional radiation partial thyroidectomy" with homogeneous thyroid irradiation is also impossible. In view of the similar radiobiologic characteristics of mammalian cells it is probable that these conclusions are relevant to the treatment of thyrotoxicosis with I131.