Coenzyme Q is an important mitochondrial redox component and the only endogenously produced lipid-soluble antioxidant. Its tissue concentration decreases with aging and in a number of diseases; dietary supplementation of this lipid would fulfill important functions by counteracting coenzyme Q depletion. To investigate possible uptake, rats were administered 12 mumol coenzyme Q10/100 g body wt once daily by gastric intubation. The appearance of coenzyme Q10 in various tissues and blood after 6 h, 4 d or 8 d was studied. The control group of rats received rapeseed-soybean oil (the vehicle in the experimental group). Lipids were extracted with petroleum ethermethanol, and the reduced and oxidized forms of coenzyme Q9 and Q10 were separated and quantified by reversed-phase HPLC. In the plasma, the total coenzyme Q concentration was doubled after 4 d of treatment. Coenzyme Q10 was also recovered in liver homogenates, where, as in the plasma, it was largely in the reduced form. Uptake into the spleen could be to a large extent accounted for by the blood content of this organ. No dietary coenzyme Q10 was recovered in the heart or kidney. The uptake in the whole body was 2-3% of the total dose. Coenzyme Q10 found in the liver was located mainly in the lysosomes. Dietary coenzyme Q10 did not influence the endogenous biosynthesis of coenzyme Q9. This is in contrast to dietary cholesterol, which down-regulates cholesterol biosynthesis. The dietary coenzyme Q10 level in the plasma decreased to approximately 50% after 4 d. These results suggest that dietary coenzyme Q10 may play a role primarily in the blood and that no appreciable uptake occurs into tissues.