We have investigated the effect of rat liver perfusion with adriamycin on mitochondrial activities. Although the perfusion treatment per se induces some decline of respiratory activities, adriamycin strongly potentiates this effect; moreover the coenzyme Q9 content of the mitochondrial membrane is significantly lowered by the antibiotic. Coaddition of coenzyme Q10 in the perfusate significantly protects the mitochondria, not only from loss of respiratory activities but also of the endogenous CoQ9 content. Exogenous CoQ10 fails to enhance respiratory activities in control rats, not treated with adriamycin, even though CoQ concentration has been proven not to be kinetically saturating in the respiratory chain under physiological conditions. Thus, the beneficial effect of CoQ10 in the perfusate does not appear to be the result of its role in the respiratory chain but is a consequence of its antioxidant action.