Fischer 344 rats on a prolonged low-iodine diet developed thyroid nodules which were transplanted subcutaneously into 33 thyroidectomized rats. One graft took and gave a papillary carcinoma. Serially transplanted syngeneically at 3to 4-month intervals, it became autonomous in third generation hosts. With electron microscopy examination it was microfollicular in year 8 and anaplastic, with totally unpolarized epithelial cells, in year 15. Basement membrane present in year I was no longer visible in specimens of years 8 and 15. Tight and gap junctions still present in year 8 were absent in year 15, although a few desmosomes persisted. From years 8 to 15, free ribosomes and microfilaments became more frequent, whereas the number of mitochondria and the development of the Golgi complex declined. male rats were fed a low-iodine diet (modified Remington) for several months according to a method previously described by Axelrad and Leblond. ' The animals developed enlarged thyroids showing hyperpla~ia'-~ of follicular epithelium and eventually nodules. I-' Fragments of these thyroids were transplanted by Mandavia and Nadler6.' under the skin of 33 thyroidectomized rats of the same inbred strain. Only one graft took, but it was successfully transplanted in all second generation animals. These were either normal rats placed on a lowiodine diet for 4 weeks prior to transplantation, or rats thyroidectomized 48 hours prior to transplantation, both groups being kept on a low-iodine diet following inoculation.6 Tumors from nonthyroidectomized rats on the low-iodine diet were successfully transplanted into all third generation hosts whether the recipients From the