Mitochondrial beta -oxidation provides much of the fuel requirements of heart and skeletal muscle despite the malonyl-CoA concentration greatly exceeding the IC50 of carnitine palmitoyl transferase for malonyl-CoA. To try to explore the relationship between inhibition of carnitine palmitoyl transferase I activity and beta -oxidation flux, we measured the flux control coefficient of carnitine palmitoyl transferase I over beta -oxidation carbon flux in suckling rat heart mitochondria. The flux control coefficient was found to be 0.08 +/- 0.05 and 50% of carnitine palmitoyl transferase I activity could be inhibited before beta -oxidation flux was affected. These observations may help to explain the presence of high rates of beta -oxidation despite the high concentration of malonyl-CoA in rat heart; we hypothesize that although not rate-limiting in vitro, carnitine palmitoyl transferase is rate-limiting in vivo because of the high malonyl-CoA concentration in heart and muscle.