Multiple cofactors and chromatin remodeling complexes have been identified to contribute to the transcriptional activation regulated by thyroid hormone receptors (TRs) in vitro. However, their role and function during development in vivo remain to be elucidated. The total dependence of amphibian metamorphosis on thyroid hormone (T3) provides a unique vertebrate model for studying the molecular mechanism of TR function in vivo. In this study, we show that the expression of BRG1, a chromatin-remodeling enzyme, is up-regulated at the climax of Xenopus laevis metamorphosis, while BAF57, a BRG1-binding protein in BRG1-containing chromatin remodeling complexes, is constitutively expressed during development. Consistently, T3-treatment of premetamorphic tadpoles led to up-regulation of the expression of BRG1 but not BAF57. Studies using a reconstituted T3-dependent Xenopus oocyte transcription system, where we could study TR function in the context of chromatin, revealed that BRG1 enhances the transcriptional activation by ligand-bound TRs in a dose-dependent manner, while a remodeling-defective BRG1 mutant inhibited the activation, suggesting that this process relies on chromatin remodeling. Further studies showed that BAF57 interacted with BRG1 in oocytes and enhanced gene activation by TR cooperatively with BRG1 in vivo. Chromatin immunoprecipitation revealed that BAF57 was recruited to the TR-regulated promoter in the presence of TR and T3. Together, these findings suggest a role of BRG1/BAF57-containing chromatin remodeling complexes in TR-regulated gene expression during postembryonic development.