In adult Anuran and neotenic Urodela, bred in laboratory conditions, the levels of plasma thyroid hormones are undetectable (T3 less than 50 ng/100 ml, T4 less than 5 ng/100 ml). Thyroid function can be reactivated after ovine TSH treatment. Under those conditions, metamorphosis is induced in the axolotl and T4 plasmatic levels reaches 0,53 +/- 0,13 micrograms/100 ml and those of T3 9 +/- 2,64 ng/100 ml. In adult Anuran, thyroid reactivation under thyrotropic treatment determines an increased secretion of T4 whereas T3 remains below the limits of detection of the assay procedure. This aptitude of adult Anuran thyroid to answer thyrotropic stimulation suggests a cyclic function of the gland after metamorphosis.