This study is concerned with the characteristics of thyroid hormone binding to isolated hepatic nuclei from premetamorphic tadpoles. Conditions essential for nuclear stability and/or demonstration of saturable binding included 220-mosmol buffers containing 0.1 mM ZnCl2 and removal of most of the melanin granules; binding of T4 and T3 to melanin was significant, but unsaturable. Scatchard analysis of [125I]T3 binding to nuclei in the presence of increasing concentrations of T3 revealed the presence of two sets of saturable sites: a high affinity, low capacity set and a second set which had a lower affinity but approximately 4 times the capacity of the first set. Two sets of T4-binding sites were also detected. The data indicate that the two hormones bind to the same two sets of sites. Thus, both T3 and T4 completely displaced either [125I]T3 or [125I]T4 bound to saturable sites, although more T4 than T3 was required for 50% displacement of either hormone. Moreover, the presence of a partially saturating concentration of T4 in a displacement study of [125I]T3 by cold T3 resulted in decreased affinity of both sets of T3-binding sites. Both sets of sites have a higher affinity for T3 than for T4. It is postulated that the high affinity set of sites consists of hormone receptors.