A method is described for determining CoQ.sub.10 concentrations in plasma samples. CoQ.sub.10 in the plasma sample is oxidized by treating the sample with an oxidizing agent having a redox potential higher than the redox potential of CoQ.sub.10, such as, for example, para-benzoquinone. Following oxidation of the CoQ.sub.10, the CoQ.sub.10 in the plasma sample is extracted with an alcohol, such as, for example, 1-propanol. The alcohol extract is analyzed using direct injection into the HPLC apparatus. This method achieves a rapid, accurate analysis of plasma CoQ.sub.10 levels, which can be used for monitoring the bioavailability of orally administered CoQ.sub.10 used as a food supplement or as an adjunctive therapy.