A UPLC-MS method for determining Coenzyme Q(10) (CoQ(10)) levels in rat serum was developed. CoQ(10) was quantitatively extracted into 2-propanol using a fast extraction procedure. The separation of CoQ(10) was performed on a Waters Acquity UPLCtrade mark BEH C(18) column (1.7 microm, 1.0 mm x 50 mm) with the mobile phase containing acetonitrile, 2-propanol, and formic acid (90:10:0.1) over 5 min. The sensitivity of this method allows for the quantitation of 50 ng/mL CoQ(10) in serum (S/N=10). The linearity of this method was found to be from 50 to 20,000 ng/mL. The precision was less than 10% (intra- and inter-day), and the average extraction recovery was between 90 and 105%. This procedure provides a precise, sensitive and direct assay method for the determination of CoQ(10) in rat serum after oral administration. This method could be applied to further pharmacokinetic studies of CoQ(10).