Thyroid hormone (T3) plays pivotal roles in vertebrate development, acting via nuclear receptors (TRs) that regulate gene transcription by promoting posttranslational modifications to histones. Methylation of cytosine residues in DNA also modulates gene transcription, and our recent finding of predominant DNA demethylation in the brain of Xenopus tadpoles at metamorphosis, a T3-dependent developmental process, caused us to hypothesize that T3 induces these changes in vivo. Treatment of pre-metamorphic tadpoles with T3 for 24 or 48...