After six-day administration of rolitetracycline (RTC) in the cardiac mitochondria of old rabbits the rate of oxygen consumption (glutamate-malate as well as 2-oxoglutarate) declines, while the malonic dialdehyde (MDA) rises. This provides evidence of more intense lipoperoxidation due to rolitetracycline. However, the ATP-ase activity increases and therefore this parameter is unsuitable in this experimental set-up (presence of tetracycline antibiotic) as an indicator of impaired integrity of mitochondrial membranes. After administration of RTC the activity of superoxide dismutase (SOD) increases markedly, but in case of concurrent administration of vitamin E it declines which is evidence of the prooxidative action of tetracyclines. Tetracycline itself and its metabolites which are formed during the biotransformation and degradation of epianhydrotetracycline (ETC) do not alter significantly the oxygen consumption nor the ATP-ase activity in cardiac mitochondria of old rabbits in vitro. Of these metabolites only anhydrotetracycline (ATC) and in particular epianhydrotetracycline (EATC) inhibit both parameters. From experiments in vitro ensues the relationship between the action and structure of the antibiotic and its metabolites the structure of cycle C and the nature of the substituents in positions C6 and C1 of this tetracycline cycle, as prerequisites are created for the formation of prooxidative structures.