The authors show the direct in vitro action of thyroid hormones on RNA-polymerase activity in rat liver mitochondria. 3,5,3' L-triiodothyronine (L-T3) and 3,5,3',5' L-tetraiodothyronine (L-T4) stimulate mitochondrial RNA synthesis without either increasing the permeability of preswollen mitochondria or stimulating the synthesis of the triphosphate ribonucleotides (NTP's). Thyroid hormones do not directly depress mitochondrial RNA hydrolysis. Studies carried out with structural analogues of thyroid hormones indicate the structural specifications of the regulating system of the mitochondrial RNA-polymerase. L-T3 and L-T4 are also effective 'in vitro' on mitochondria obtained from animals undergoing different hormonal and dietary treatments, with the exceptions of those fed with a hypoprotein diet. Thus, the authors suggest the possible intervention of a specific mitochondrial receptor for L-T3 and L-T4.