Thyroid hormones regulate tissue metabolism establishing an energy balance in the cell, in particular by affecting oxidative phosphorylation. Their long-term impact is mainly associated with changes in gene expression, while the short-term effects may differ in mechanism. Our work is devoted to short-term effects of hormones T2, T3, and T4 on mitochondrial cytochrome c oxidase (CcO) mediated by a direct contact with the enzyme. The data obtained indicate the existence of two separate sites of CcO interaction with thyroid hormones differing in location, affinity and specificity to hormone binding. It is shown that T3 and T4 but not T2 inhibit oxidase activity of CcO in solution and on membrane preparations with Кi≈100–200 M. In solution, T3 and T4 compete in a 1:1 ratio with the detergent dodecyl-maltoside for binding to the enzyme. Peroxidase and catalase partial activities of CcO are not sensitive to hormones while electron transfer from heme a to the oxidized binuclear center is affected. We believe that T3 and T4 are ligands of the Bile Acid Binding Site found in the 3D structure of CсO by Ferguson-Miller’s group, and hormone induced inhibition is associated with dysfunction of the K- proton channel. Similar conclusion we made recently with regard to steroid-like compounds. It is found that T2, T3, and T4 inhibit superoxide generation by oxidized CcO in the presence of excess Н2О2. Inhibition is characterized by Ki values of 0.3 – 5 M and apparently affects the formation of О2• at the protein surface. The second binding site for thyroid hormones presumably coincides with the point of tight T2 binding on the Va subunit described in the literature.