The level of thyrotropin stimulation of rat thyroid was modified to permit a study of the regulation of some of the enzymes of this tissue. This was accomplished by the administration of either propylthiouracil to increase the endogenous thyrotropin levels or thyroxine to suppress production of the trophic hormone. The enzymes measured included two glycosyltransferases involved in the synthesis of the main secretory protein of the gland (thyroglobulin), two lysosomal enzymes which may contribute to its catabolism, as well as two other enzymes of a more general nature. In the propylthiouracil-treated animals changes in the activities of succinic dehydrogenase, protease, and 5′-nucleotidase corresponded to the increase in the weight of the gland and appeared to be nonspecific in nature; these three enzymes, moreover, showed no changes after thyroxine treatment. The level of the N-acetylglucosaminidase was significantly decreased, per gram of tissue, in both groups of treated animals. The only enzymes which appeared to be specifically affected by the modulation of thyrotropin stimulation were the glycosyltransferases, with both mannosyl- and galactosyltransferases showing an increase in the thyroids of the propylthiouracil-treated animals and a decrease in those treated with thyroxine. This suggests that post-translational steps, such as carbohydration, may play an important role in regulating the turnover of thyroglobulin and therefore influence the overall rate of thyroid hormone formation. The distribution of each of the enzymes between the soluble and particulate fractions of the tissue was also measured and it was noted that the glycosyltransferases, which showed the most marked increase in total activity as a result of thyrotropin stimulation, also showed a statistically significant increase in the percentage present in the particle-bound form.