The effect of the thyroid hormone on the rate of brain protein synthesis in rats was studied. Experiments were conducted on three groups of rats given 6-propyl-2-thiouracil (PTU, a thyroid inhibitor) without a triiodothyronine (T3) treatment, those treated with PTU + T3, and those treated with neither PTU nor T3 (control). The fractional rates of protein synthesis in the brain, liver, and kidney of rats given PTU + T3 were significantly greater than those in rats given PTU alone. In the brain and kidney, the RNA activity [g of protein synthesized/(g of RNA.d)] were significantly correlated with the fractional rates of protein synthesis. In the liver and kidney, the RNA concentration (mg of RNA/g of protein) was related to the fractional rate of protein synthesis. These results suggest that the thyroid hormone treatment would be likely to increase the rate of protein synthesis in the brain of rats, and that the RNA activity is, at least partly, related to the fractional rate of brain protein synthesis.