Male rats, 5 and 90 d of age, continuously inhaled 500 ppm CO [40% carboxyhemo‐globin (COHb)l for 5.5–8.0 wk. Cardiomegaly andpolycythemia developed as previously reported. Heart weight gain in young and old rats treated with CO was accompanied by increased cytochrome c content (nmol) in both left ventricle (LV) and right ventricle (RV) relative to controls. Cytochrome c concentration (nmolIg wet weight), however, was significantly depressed in LV and RV of young CO‐exposed rats relative to controls, while there was no change in LV and RV of the old CO‐exposed group. LV cytochrome c concentration was significantly higher than that of RV in both young and old, CO‐exposed and control rats. On the other hand, cytochrome c concentration in young and old control LVs was similar, as was cytochrome c concentration in RVs of young and old control rats. Three additional experiments were carried out with 5‐d‐old rats inhaling 500 ppm CO, for 47 and 25 d. These also showed the increase in myocardial cytochrome c content and compromise of cytochrome c concentration, and both changes were greater with longer exposure.