Comparative and competitive analyses of thyroxine (T4) and triiodothyronine (T3) 
binding to highly purified rat liver, brain and lung cell plasma membranes were 
carried out. The dependence of hormone binding on the time, temperature and 
concentration was studied. The effects of trypsin and partial delipidation on 
the binding parameters of thyroid hormones were investigated. Two thyroid 
hormone-binding sites were detected in cell plasma membranes of all tissues 
under study. The maximal binding of T4 to rat liver membranes and the maximal 
binding of T3 to rat brain membranes was observed in all experiments, the 
affinity for T3 being higher than that for T4. An important role of both protein 
and lipid components of plasma membranes in the membrane reception of thyroid 
hormones is proposed.