The specific binding of 125I-thyroxine and 125I-triiodothyronine in rat liver nuclei and 0.4 mol 1(-1) KCl nuclear extract is presented. The Scatchard plot analysis for triiodothyronine calculated from the competition curve of isolated nuclei gives Ka = 1.3 x 10(9) mol-1 and from that of nuclear extract gives Ka in the range of 10(8)--10(10) 1 mol-1. Crude nuclear extract of rat liver nuclei (0.4 mol 1(-1) KCl, 5 mmol 1(-1) dithiothreitol) after removing of inorganic KCl on G-25 Sephadex was fractionated on G-100 Sephadex. Two different nuclear protein fractions were obtained and the distribution of 125I-thyroxine and 125I-triiodothyronine in these protein components was tested. 125I-thyroxine binds clearly on both protein fractions, whereas 125I-triiodothyronine binding was found only in the major protein component. The fractionation of nuclear extract incubated with labelled thyroid hormones with or without the excess of 0.04 nmol of thyroxine or triiodothyronine demonstrate a specific binding of both thyroid hormones to major protein component and nonspecific binding of thyroxine to a fraction representing the minor protein component.