Thyroxine (T₄) and 3,5,3'-triiodothyronine (T₃) were measured by radioimmunoassay in plasma and thyroid of Xenopus laevis tadpoles during pre-, prometamorphosis and climax. The plasma level of both hormones increased at the start of climax to reach a maximum at the mid climax. Furthermore T₃ was distinctly lower than the T₄ before climax but surpassed it at the mid climax. lt is likely that the T₃ increase during the climax results essentially from T₄ peripheral deiodination rather than from an increase in thyroidal T₃ secretion, as the thyroglobulin T₃/T₄ ratio was constantly very low. T₃ thus appears to be the hormone of metamorphosis.