Three anuran species (Rana perezi, Xenopus laevis, and Bufo calamita) of different phylogenetic origins and ecological habitats have been studied during ontogeny with respect to day/night changes in whole-body concentrations and total content of extrathyroidal thyroxine (T4) and triiodothyronine (T3). There were no significant day/night changes in thyroid hormones (TH) during embryonic stages. Daily cycles in TH with higher nocturnal values appeared during premetamorphosis in R. perezi and X. laevis. Cyclicity disappears for T3, while it is reversed for T4, in prometamorphic R. perezi and X. laevis. In contrast, there were significantly higher T3 (0.74 +/- 0.13 ng/g) and T4 (2.08 +/- 0.54 ng/g) levels at night in prometamorphic B. calamita. Significant daily changes in T3 and T4 with higher nocturnal values (T3, 788.29 +/- 118.38 pg/g; T4, 1.95 +/- 0.4 ng/g) were again seen in X. laevis at the end of climax, while in B. calamita low TH values appeared at early scotophase and there were no significant changes in R. perezi at this time. Similar daily profiles were observed for TH whole-body concentrations and total contents.