Thiouracil, 2-thio-6-aminouracil and an iodine-deficient diet produced an enlargement of rat thyroid glands with a subsequent increase in the acid-soluble nucleotides and RNA per unit wet weight of tissue, while DNA content remained constant or decreased slightly. No change was observed in liver. The pattern of RNA nucleotides of both thyroid and liver was comparable in thiouracil- treated and untreated rats. Uridine phosphorylase and uridine kinase were both more than twice as active in thyroid glands from thiouracil-treated rats as compared to normal glands, while no difference in the activities of the 2 enzymes was observed in liver. Both uracil-2-14C and orotic acid-6-14C were incorporated into CMP and UMP of liver and thyroid RNA from treated and untreated animals. Uracil-2-14C incorporation was 3 times greater in thyroid tissue than in liver, but orotic acid-6-14C was only slightly higher in thyroid. Uracil-2-14C and orotic acid- 6-14C incorporation was increased in goitrous thyroid tissue. The increase was quantitatively similar to the increase in RNA content of the gland resulting in comparable specific activities of CMP and UMP in the isolated RNA. The major radioactive compounds identified after incubation of uracil-2-14C with liver and thyroid slices were UMP, a UDP conjugate, UDP and UTP. After incubation of orotic acid-6-14C, UMP, 2 UDP conjugates, UDP and UTP were found. (Endocrinology76: 737, 1965)