The main objective of this study was to resolve the issue of whether the amounts of Coenzyme Q (CoQ), which is endogenously synthesized in cells, can be elevated in tissues and mitochondria of young mice by dietary supplementation with CoQ10. The prevalent view is that the uptake of exogenous CoQ by tissues other than plasma and liver either does not occur or is quite minimal. Mice, 6 mo of age, were fed 0, 148 or 654 mg CoQ10/(kg body x d) in their diets for 11 wk. CoQ10 intake enhanced both CoQ9 and CoQ10 homologues in the plasma, and in homogenates and mitochondria of liver, heart and skeletal muscle. CoQ was elevated in brain mitochondria, but not in the brain homogenate. The uptake of exogenous CoQ was higher in mitochondria of heart and skeletal muscle than those in liver. CoQ10 administration also elevated the alpha-tocopherol concentration in tissue homogenates and their mitochondria, thereby providing an in vivo indication of the "sparing" effect of CoQ on alpha-tocopherol. Results of this study demonstrate that, contrary to the historical view, both total and mitochondrial CoQ concentrations in the heart and skeletal muscle and in the mitochondria of brain of young mice can be augmented by dietary supplementation. Furthermore, CoQ intake enhances the antioxidative potential of tissues by elevating the endogenous amounts of alpha-tocopherol.