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The role of T4 as an active hormone in premetamorphic tadpoles has been investigated. It was found that the only labeled compounds present in serum and preparations of liver cytosol and nuclei 1, 3, and 24 h after injection of [125I]T4 and [125I]T3 (10 pmol/tadpole) were iodide, the injected hormone, and a trace of labeled material that remained at the origin during chromatography (origin material). Although T4 was deiodinated more rapidly than T3, no conversion of T4 to T3 was detected. Similar observations were made 72 h after the administration of 10 nmol of either hormone, at which time hepatic carbamyl phosphate synthetase activity was significantly increased. Distribution and turnover of [125I]T3 and [125I]T4 (10 pmol) administered ip were determined in premetamorphic tadpoles. Maximum concentrations of both hormones in serum and liver occurred at 2.5 h. In serum, the concentration of T4 exceeded that of T3 at 1 h, but by 3 h and at all subsequent time intervals studied, T3 concentration was higher than that of T4. In liver, the uptake of T4 during the first 6 h was double that of T3. Moreover, when tadpoles were given both hormones simultaneously in equimolar amounts, at least twice as much T4 as T3 was present in both liver cytosol and nuclei after 3 and 24 h. The levels of both hormones in serum and liver decreased rapidly after 2.5 h, and these decreases were accompanied by an increase in the amount of 125I in bile. It was concluded that T4 can exert a physiological effect in premetamorphic tadpoles without being converted to T3. Furthermore, the data suggest that, relative to T3, T4 plays a more important role as an active hormone in premetamorphic tadpoles than it is thought to play in mammals.