Thyroid hormone is essential for brain development. L-thyroxine (T4) has to be converted to triiodothyronine (T3) for efficient binding to thyroid hormone receptors. Type 2 deiodinase (D2) is the key enzyme that allows T3 generation in the brain. To elucidate the onset and localization of T3 production in the brain, we have studied the changes of D2 activity, mRNA content and the distribution of D2 mRNA in the brain of chicken embryos before and after the onset of thyroid function. D2 activity was detectable in the brain at all stages studied from E7 to E15 and increased significantly with time. The wild-type chicken D2 transcript was detectable at all those stages by RT-PCR. The amount of D2 mRNA in the brain increased 14 fold from E10 to E17 as assessed by Northern blot. Week D2 hybridization signal could be detected by in situ hybridization at E8 in cell clusters throughout the brain and its intensity markedly increased to E15. Interestingly, no D2 expression was detected in hypothalamic tanycytes at these embryonic stages. However, D2 hybridization signal was observed in the wall of the third ventricle of adult chicken posterior to the rostral pole of the median eminence in the location typical for tanycytes while D2 signal in other localizations was decreased throughout the brain. Our data suggest that D2 contributes to T3 content of the developing chicken brain even before the onset of thyroid function. Furthermore, redistribution of D2 mRNA expression was observed during the development of the chicken brain.