The pharmacokinetics of coenzyme Q10 (CoQ10) in man was studied by utilizing deuterium-labelled coenzyme Q10 (d5-CoQ10). The absence of an isotope effect in the disposition of d5-CoQ10 in man was confirmed from the plasma concentration time curves after simultaneous oral dosing of d5-CoQ10 and unlabelled CoQ10. After oral administration of 100 mg of d5-CoQ10 to 16 healthy male subjects, the mean plasma CoQ10 level attained a peak of 1.004 +/- 0.370 micrograms/ml at 6.5 +/- 1.5 h after administration, and the terminal elimination half-life was 33.19 +/- 5.32 h. In most of the subjects, plasma d5-CoQ10 showed a second peak at 24 h after dosing. This unusual plasma level curve was well described by a newly proposed compartment model based upon the assumption that absorbed CoQ10 is taken up by the liver and then transferred mainly to VLDL and redistributed from the liver to the systemic blood.