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Amphibian metamorphosis represents a unique biological model to study thyroid hormone (TH) action in vivo. In this study, we examined the utility of thyroid hormone receptors α (TRα) and βA (TRβA) mRNA expression patterns in Xenopus laevis tadpoles as molecular markers indicating modulation of TH action. During spontaneous metamorphosis, only moderate changes were evident for TRα gene expression whereas a marked up-regulation of TRβA mRNA occurred in hind limbs (prometamorphosis), head (late prometamorphosis), and tail tissue (metamorphic climax). Treatment of premetamorphic tadpoles with 1 nM 3,5,3′-triiodothyronine (T3) caused a rapid induction of TRβA mRNA in head and tail tissue within 6 to 12 h which was maintained for at least 72 h after initiation of T3 treatment. Developmental stage had a strong influence on the responsiveness of tadpole tissues to induce TRβA mRNA during 24 h treatment with thyroxine (0, 1, 5, 10 nM T4) or T3 (0, 1, 5, 10 nM). Premetamorphic tadpoles were highly sensitive in their response to T4 and T3 treatments, whereas sensitivity to TH was decreased in early prometamorphic tadpoles and strongly diminished in late prometamorphic tadpoles. To examine the utility of TRβA gene expression analysis for detection of agonistic and antagonistic effects on T3 action, mRNA expression was assessed in premetamorphic tadpoles after 48 h of treatment with the synthetic agonist GC-1 (0, 10, 50, 250 nM), the synthetic antagonist NH-3 (0, 40, 200, 1000 nM), and binary combinations of NH-3 (0, 40, 200, 1000 nM) and T3 (1 nM). All tested concentrations of GC-1 as well as the highest concentration of NH-3 caused an up-regulation of TRβA expression. Co-treatment with NH-3 and T3 revealed strong antagonistic effects by NH-3 on T3-induced TRβA mRNA up-regulation. Results of this study suggest that TRβA mRNA expression analysis could serve as a sensitive molecular testing approach to study effects of environmental compounds on the thyroid system in X. laevis tadpoles.