The role of tryptophane and arginine residues of rat liver receptors for the specific binding of 3,5,3'-triiodothyronine (T3) was studied by chemically modifying the receptor molecule. Soluble T3 receptor fraction was prepared from purified rat liver nuclei and the kinetics of the modification of a tryptophane indol ring of nuclear receptor by N-bromsuccinimide (NBS) in the presence of excess -SH protecting agent was examined. Moreover the kinetics of the formation of N5-(4-oxo-1,3-diazospiro[4,4]non-2-ylidene)-I-ornithine or N7,N8-(1,2-dihydroxycyclohexyl-1,2-ylene)-L-arginine from arginine residue(s) of nuclear receptor by 1,2-cyclohexanedione was investigated. The efficiency of the reactions were followed spectrophotometrically and the modified nuclear receptor fraction separated from chemical modifiers on a Sephadex G-25 column was assayed at pH 8.0 for T3 specific binding. The T3 specific binding was tested by Scatchard plot analysis. No changes in nuclear receptor Ka or MBC were observed after 1,2-cyclohexanedione treatment. Tryptophanyl residue(s) of the receptor molecule may play an effective role in the maintaining the nuclear receptor in a conformation optimal for T3 binding.