The ability of thyroid slices to incorporate inorganic phosphate into organic P compounds is comparable to that of kidney and liver. Homogenization of the thyroid tissue before incubation prevents this synthesis. In experiments where thiouracil or thyroxine were added to the incubating thyroid slices, the incorporation of radioactive phosphate into organic compounds was not affected. Azide in 0.005 [image] concentration was, however, an effective inhibitor of the process.