Presence of a thyroxine-binding protein was demonstrated in vivo in cell sap of tail and liver of metamorphosing Rana catesbeiana tadpoles. Thyroxine-binding protein was not present in tail of prematamorphic tadpoles while it appeared during progressing metamorphosis roughly coinciding with the beginning of tail resorption. Susceptibility to pronase indicates that this thyroxine-binding macromolecule is protein in nature. Thyroxine-binding in liver was already present during premetamorphic stages and increased further during metamorphosis. A further difference between tail and liver thyroxine-binding protein was evidenced by molecular sieve chromatography on Sephadex G-200 indicating a molecular weight of thyroxine-binding protein in the tail of 60 000 as opposed to 42 000 for liver. Scatchard analysis of tail cell sap of tadpoles in metamorphic climax revealed a high affinity thyroxing binding site (Kd of 2 - 10(-10) M) of low capacity (1.7 pmol per mg protein) while tadpoles in premetamorphic stage had a thyroxine-binding site of lower affinity (9 - 10(-10) M) and higher capacity (4.8 pmol per mg protein). Thus affinity of thyroxine binding is 4-fold in metamorphic climax and appears to reflect the appearance of thyroxine binding observed in vivo.