Brain protein synthesis and the plasma concentration of growth hormone (GH) are sensitive to dietary ornithine. However, dietary ornithine does not increase brain protein synthesis in hypophysectomized rats. Because hypophysectomy may decrease the secretion of thyroid stimulated hormone (TSH), we assessed whether the regulation of brain protein synthesis was mediated by changes in the plasma concentrations of thyroid hormone and ghrelin in the 6-propyl-2-thiouracil (PTU, thyroid inhibitor)-treated or control adult rats fed ornithine. The four experimental groups consisted of PTU-treated and control (24-wk-old) male rats given 0% or 0.7% ornithine-HCl added to a 20% casein diet. The plasma concentrations of GH and ghrelin, and the fractional rates of protein synthesis and RNA activity [g protein synthesized/(g RNA•d)] in the brains were significantly increased after treatment with the 20% casein + 0.7% ornithine compared with the 20% casein diet alone in both the PTU-treated and control groups. Ornithine supplementation to the basal diet did not affect the plasma concentration of T₃. The RNA concentration (mg RNA/g protein) was not related to the fractional rate of protein synthesis in the brain regions. The results suggest that dietary ornithine likely increases the rate of brain protein synthesis in control and PTU-treated rats, and that the ornithine-induced increase in the GH concentration may stimulate mainly brain protein synthesis via ghrelin. RNA activity is at least partly related to the fractional rate of brain protein synthesis.