Coenzyme (Co) Q10, Co Q10H2, alpha-tocopherol and cholesterol were dissociated from lipoproteins in plasma by treatment with 1-propanol. The supernatant obtained was injected directly for determination of Co Q10 and Co Q10H2. Precolumn reduction with borohydride was used for determination of total Co Q10 simultaneously with alpha-tocopherol and cholesterol. Total Co Q10 in freeze-dried myocardial biopsies was determined after extraction with 1-propanol and oxidation of Co Q10H2 with ferric chloride. The chromatographic system comprised two reversed-phase columns and a three-electrode coulometric detector and a UV detector coupled in series. A pre-fractionation on the first column protected the coulometric detector from contamination and reduced the time for analysis by eliminating strongly retained solutes. The coulometric electrodes were operated in the oxidation-reduction-oxidation mode, and the last electrode was used for detection of alpha-tocopherol, Co Q10 and Co Q10H2, while cholesterol was detected by UV at 215 nm. The fast isolation procedure made it possible to determine the reduced and oxidized forms of Co Q10 in plasma. Quantitative recoveries were obtained for all the analytes studied and normal levels were determined with a coefficient of variation of 2-3%.