Brain development in mammals is dependent on thyroid hormone. In the rat, the T3 receptor is present as early as the 14th day of fetal life and increases during the periods of neuroblast proliferation and oligodendrocyte differentiation. The mRNAs encoding the receptor isoforms can also be demonstrated by in situ hybridization, with different distribution of the alpha or beta forms. Expression of several genes was studied in hypothyroid animals during the neonatal period. The lack of thyroid hormone affects the expression of most oligodendrocyte genes in a time- and region-dependent fashion. The major role of thyroid hormone on myelin genes is on their timing of expression. Among the neuronal genes studied, thyroid hormone influences the expression of RC3, a brain specific gene encoding a protein kinase C substrate. Thyroid hormone is not require for the timing of expression of this gene, but is needed for its full expression both during the neonatal period and in adult animals.