Serum and urinary levels of ubiquinol-10 (UQH2-10) as well as total ubiquinone-10 (total UQ-10, sum of UQH2-10 and oxidized UQ-10) were determined in healthy Japanese subjects using high performance liquid chromatography with an electrochemical detector. The mean serum and urinary levels of UQH2-10 were 0.75 +/- 0.21 microgram/ml (86% of total UQ-10, n = 77) and 0.045 +/- 0.016 microgram/mg creatinine (59% of total UQ-10, n = 30), respectively. After daily oral administration of 60 mg of UQ-10 for 7 days, the human serum level of UQH2-10 increased twice as compared to that before the treatment. However, the ratio of the UQH2-10 to the total UQ-10 remained unchanged during such administration. The serum level of UQH2-10 correlated significantly with those of vitamin E (p less than 0.001), phospholipids (p less than 0.01), total cholesterol (p less than 0.05), and total lipids (p less than 0.05). There was no correlationship between the serum levels of UQH2-10 and of triglycerides and lipid peroxides.