Thyroidectomy reduced the incorporation of [3H]amino acids into total rat pituitary proteins as well as into electrophoretic fractions corresponding to growth hormone (GH) and prolactin. A single injection of thyroxine (200 microgram/kg) reversed the effects of thyroidectomy on the GH (12 h) and prolactin (36 h) fractions. Evidence for a general increase in the rate of pituitary protein synthesis following this treatment was less conclusive, but two injections of the hormone had a definite stimulatory effect. [3H]Uridine uptake by the pituitary and its incorporation into RNA were elevated for 2 weeks after thyroidectomy. Thyroxine failed to suppress these changes for at least 36 h after injection. Actinomycin C (400 microgram/kg) inhibited pituitary RNA synthesis and blocked the stimulatory effect of thyroxine on amino acid incorporation into the GH fraction. These results indicate that thyroid hormones: (1) promote pituitary protein synthesis, probably as a consequence of their effects on somatotrophs and lactotrophs; (2) exert a stimulatory effect on GH synthesis by affecting transcription.