Many of the polypeptides made on endogenous ribosomes inside of yeast mitochondria are hydrophobic "integral polypeptides" which are subunits of at least three oligomeric enzyme complexes (cytochrome c oxidase, rutamycin-sensitive ATPase, and coenzyme QH2-cytochrome c reductase) of the inner mitochondrial membrane. In order to elucidate the pathway(s) followed by these polypeptides into the inner membrane we have used an in vitro mitochondrial translation system from yeast. By inhibiting this system with aurintricarboxylic acid, we have been able to demonstrate and accumulate a transient precursor to subunit II of cytochrome c oxidase. This precursor, designated II', is approximately 1,500 daltons larger than mature subunit II and most likely is a form of subunit II with an NH2-terminal extension. Although this precursor appears to be processed cotranslationally under normal conditions, it does associate in unprocessed form with mitochondrial membranes when allowed to accumulate in the presence of aurintricarboxylic acid, and it can be processed postranslationally upon removal of the drug. None of the other mitochondrial translation products made in this system exhibits larger precursors. These results indicate that at least one mitochondrial translation product has a transient "leader sequence" a,d is inserted into the inner mitochondrial membrane and processed cotranslationally, but they suggest that other pathways may be followed by the other translation products.