The stimulation of respiration by long-chain fatty acids and FCCP was studied with oligomycin-inhibited mitochondria from rat liver, heart and kidney tissue. By addition of equal amounts of palmitate and oleate, mitochondrial respiration was increased in the order RLM less than RKM less than RHM. Using the classical protonophore FCCP, this difference could not be observed. Inhibition of oleate-stimulated respiration by carboxyatractyloside decreased in the order RHM greater than RKM greater than RLM. As CAT sensitivity of oleate-stimulated respiration and the mitochondrial ANT content were found to be correlated, it is suggested that the weak CAT sensitivity of oleate-stimulated respiration of RLM [(1989) Biochim. Biophys. Acta 977, 266-272] is due to the low content of ANT.