The action of TSH on protein turnover in various subcellular fractions has been investigated in dog thyroid slices incubated in vitro. The results suggest a general inhibition by TSH of protein catabolism. Using double labeline (3/ and 14C) of the proteins, an increase of the disappearance of some labeled material from the microsomal fraction in the presence of TSH has been observed. The protein nature of this material has been established by testing its susceptibility to hydrolysis by trypsin. The fact that the microsomal pellet had to be treated by triton X 100 before hydrolysis by trypsin could occur, suggests that the material is probably enclosed in, or protected by membrane vesicles. Its high molecular weight and its ability to be immunoprecipitated by an antithyroglobulin serum suggest that the microsomal protein, the disappearance of which is stimulated by TSH, is thyroglobulin or one of its subunits. It is suggested that our results reflect the acceleration by TSH of the vectorial transfer of thyroglobulin through the membranes of the endoplasmic reticulum to the colloid space.