We have investigated the effect of ageing and of adriamycin treatment on the bioenergetics of isolated rat hepatocytes. Ageing per se, whilst being associated with a striking increase of hydrogen peroxide in the cells, induces only minor changes on mitochondrial functions. The adriamycin treatment induces a decrease of the mitochondrial membrane potential in situ and a consistent increase of the superoxide anion cellular content independently of the donor's age, whilst the hydrogen peroxide is significantly higher in aged than in adult rat hepatocytes. Kinetic studies in isolated mitochondria show that the mitochondrial respiratory chain activity (NADH --> O2) of 50 microM adriamycin-treated hepatocytes is lowered both in adult and aged rats. The same adriamycin concentration induces a slight decrease of the maximal rate of ATP hydrolysis in both young and aged rats, without affecting the Km for the substrate. However, at drug concentrations lower than 50 microM, both ATPase and NADH oxidation activities decrease significantly in aged rats only. The results suggest that free radicals increase during ageing in rat hepatocytes but are unable to induce major modifications of mitochondrial bioenergetics. This contrasts with the damaging effect of adriamycin, suggesting that some effects of the drug may be due to other reasons besides oxidative stress.