The effects of dietary supplementation with Coenzyme Q10 (CoQ10), a reputed performance enhancer and antioxidant, on physiological and biochemical parameters were examined. Ten male bicycle racers performed graded cycle ergometry both before and after being given 100 mg per day CoQ10 or placebo for 8 weeks. Analysis of variance showed a significant difference between groups for postsupplementation serum CoQ10. Although both groups demonstrated training related improvements in all physiological parameters over the course of the study, there were no significant differences between the two groups (p > .05). Both groups showed a 21% increase in serum MDA (an index of lipid peroxidation) after the presupplementation exercise test. After 8 weeks this increase was only 5%, and again was identical for both groups. Supplementation with CoQ10 has no measurable effect on cycling performance, VO2max, submaximal physiological parameters, or lipid peroxidation. However, chronic intense training seems to result in marked attenuation of exercise-induced lipid peroxidation.