The effect of simvastatin administered for 10 days on coenzyme Q and antioxidant/oxidant balance in a rat model of diabetes mellitus and hypercholesterolaemia was studied. In the diabetic-hypercholesterolaemic rats the signs of oxidative stress-decreased alpha-tocopherol/cholesterol in the plasma (p < 0.01) and alpha-tocopherol in liver (p < 0.001) together with increased lipid peroxidation in the liver (TBARS, p < 0.05) were found. Increased coenzyme Q9 concentrations in the plasma (p < 0.05) and liver (p < 0.01), coenzyme Q10 in the myocardium (p < 0.05) and in the liver (p < 0.01) may indicate adaptation to oxidative stress. Administration of simvastatin (10 mg/kg) to the diabetic-hypercholesterolaemic rats counteracted increased myocardial (coenzyme Q10, p < 0.05) and liver (total coenzyme Q9, p < 0.05) coenzyme Q concentrations but did not improve alpha-tocopherol depletion and increased formation of TBARS in the liver. Even though simvastatin treatment did not induce coenzyme Q deficiency in plasma, heart and liver of the diabetic-hypercholesterolaemic rats as compared to the control levels, it was not able to prevent completely the changes in antioxidant/oxidant balance induced by diabetes and hypercholesterolaemia. The results highlight the importance of studying the effect of statins on the coenzyme Q levels in the animal models of pathological conditions known to change the initial antioxidant defence system.