Isolated rat liver mitochondria were labeled in vitro with L-[14C]leucine. Sixty percent of the incorporated radioactivity was found to reside in subunits 1, 2, and 3 of cytochrome c oxidase with apparent molecular weights of approximately 33,000, 25,000, and 20,000, respectively. The results indicate that these are the predominant products of protein synthesis under the conditions employed. The enzyme complex, as derived by immunoprecipitation, was found to contain four additional polypeptides with apparent molecular weights of 17,000, 12,500, 7000, and 3500. A comparison of electrophoretic profiles of the rat liver and beef heart enzyme reveals that the apparent molecular weights of all polypeptides are remarkably similar.