Nuclei were prepared by sucrose sedimentation of liver homogenates from rats given (125)I-labeled triiodothyronine in vivo. The nuclear extract obtained by treatment of the nuclear pellet with 0.4 M KCl contains the [(125)I]triiodothyronine that had been injected in vivo bound to protein(s). The triiodothyronine bound to nuclear protein(s) in vivo does not readily exchange with triiodothyronine added to the extract in vitro. This triiodothyronine.nuclear extract complex retains triiodothyronine during dialysis or exposure to anion exchange resin and migrates as a broad band on agarose-gel electrophoresis. It is rapidly destroyed by Pronase, by 8 M urea, and by p-chloromercuribenzoic acid, but not by RNase or by DNase. It is also susceptible to thermal inactivation at 37 degrees , possibly through changes in the affinity of triiodothyronine to the nuclear binding protein(s), since the bound triiodothyronine becomes more readily dialyzable, is absorbed by an anion exchange resin, but retains its characteristic mobility on electrophoresis. The triiodothyronine.nuclear extract complex formed in vivo binds to crude liver chromatin in vitro at low salt concentration, but can be completely extracted again at KCl concentrations greater than 0.2 M.