When leucine-C14 was infused via the carotid artery directly into the thyroid gland of intact, anesthetized rabbits, the C14 was readily incorporated into thyroid protein. Inclusion of approximately 10-3 [image] puromycin in the infused solution consistently inhibited thyroid protein formation about 95%. In I131-injected rabbits, whose thyroids were continuously exposed to puromycin infused via the carotid, TSH (thyroid-stimulating hormone) was injected peripherally, and its effect on I131-thyroxine release from the thyroid was compared with that observed in saline-infused animals. The response to TSH was unaffected in the puromycin-blocked rabbits. These observations suggest that TSH-induced thyroxine release from the thyroid gland does not depend on prior stimulation of protein synthesis. This particular action of TSH, therefore, probably does not involve stimulation of messenger RNA synthesis.