Thyroid hormone effects on the developing rat brain were analyzed regarding the nucleic acid contents and the rate of protein synthesis using PTU induced neonatal hypothyroid rat as a model for human congenital hypothyroidism. Significantly reduced amount of total DNA contents per brain and protein/DNA ratio were demonstrated in both cerebrum and cerebellum of the hypothyroid rat after 14 days, suggesting the disturbed cell formation and cell growth in the hypothyroid rat brain. And the maturation of the cerebellum extended later into postnatal life than does that of the cerebrum. Additional evidence by the reduced rate of protein synthesis in the hypothyroids would correlate with these altered morphogenesis. Postmitochondrial supernatant (PMS) obtained from both control and hypothyroid brain homogenate, which was used for the experiment of protein synthesis, contained the same amount of the RNA contents. Therefore the defective protein synthesis might be attributed to the disturbed translational process of the genetic message in protein.