We have analyzed the expression pattern of c-erbAα and c-erbAβ which encode the thyroid hormone receptors (T3Rα and T3Rβ) during early chicken embryogenesis. Only c-erbAα expression was detected by RT-PCR and whole-mountin situhybridization. c-erbAα transcripts were found to be already present at low level in embryos before egg incubation. During neurulation a marked increase was observed in neurectoderm. A reporter cell line was then constructed and used to demonstrate the release of significant amount of thyroid hormone (T3) from egg yolk by area opaca cells before gastrulation. During gastrulation T3 was found to be enriched in the primitive streak and Hensen's node. Introduction of excess T3 frequently resulted in abnormal development of anterior structures, mainly neural tube defects and anencephalia. These observations suggest that T3Rα, like the closely related retinoic acid receptors, fulfills functions which are important for embryonic development well before the onset of thyroid gland function.