Administration of the thyroid hormone 3,3,5'-triiodo-L-thyronine (T3) to rats leads to a marked increase in hepatic levels of mRNA for cytochrome c. Messenger RNA prepared from the free polysomes of T3-treated rats directed the in vitro synthesis of a polypeptide which only differed in amino acid sequence from mature cytochrome c in that it contained an NH2-terminal methionine. The in vitro product was incorporated specifically into purified rat liver mitochondria and became inaccessible to added trypsin when the mitochondria were added after translation was completed. Horse heart apocytochrome c, but not the holocytochrome, could compete with the in vitro synthesized polypeptide for its uptake into mitochondria. This suggests that the primary structural features of apocytochrome c, which serve as an addressing signal for mitochondria, are masked after the acquisition of heme and that this process occurs in the mitochondria. The addressing signal seems to be contained in a specific segment of the cytochrome polypeptide because only one fragment generated by CNBr cleavage of horse apocytochrome c, extending from residue 66 to the carboxy end of the molecule, could compete with the in vitro product for its transfer into mitochondria.