- 12 - a. . . . Several months Tollowins a modest dose of 131 T to rats anaa latent pericl 2 which Sew cells can be stimulated to synthesize DNA, a supra-maximal fsurge by -any cet? synthesizing DNA was produced by stimulating such a gland. This occurred before many of the celis had succumbed and the gland had begun to shrink in size. It was these and other related observations that led to further fexperiments in which progressively smaller and smaller doses of 131 I were used in large groups of rats. With each succeeding experiment which required 2 to 3 years tq@ neoplasms developed. complete, more The need for longer periods of observation bec4me apparent. The neoplasms first began to appear at 14 to 16 months. When there fas reasonable expectation that neoplasms were soon to appear, many animals were sSafrificed and auto- radiographs were prepared by giving a trace amount of 1 3 ty just befofe sacrifice to locate sites of fumction and tritiated thymidine (7H-TaR) to locate thesizing DNA. Fhe nuclei syn- With longer periods of waiting, neoplasms in all stapes of develonment were found and studied. Three types of lesions commonly seen in these animals were papillary, follicular and solid cellular. At least some of the lesions that develop spontaneously in elderly rats were parafollicular cell (medullary) thype as described by Lindsay and by Williams. The remainder of the solid cellular legions were rapidly growing as implied from the abundant labelling of the nuclei by 33-Far. The details of the development of the lesions have been descriffed in a manuscript which was submitted for publication but publication has been delaye@ described later. When large numbers of animals were sacrificed at for reasons to be fhe time neoplasms could be expected to be developing and thereafter for months, the sfquence of the development of neoplasms could be studied. The multicentricity of f~esions in the sane animal and the lesions found in multiple animals sacrificed at intervals permittedan interpretation of the development of the lesions. cular epithelium. Papillary lesi arose from folli- Occasional follicles several times the diameter Pf the surrounding normal follicles appeared. This was the first sign of a localized abnormality develop