The effect of carnosine (15 mM) on the contractile activity of isolated rat hearts contracting in an isotonic regime (37 degrees C at a 5 Hz stimulation frequency) has been studied. Carnosine added to the perfusing solution had no effect on the contractile activity either in hypoxia or during reoxygenation but decreased it with a simultaneous increase in the coronary flow during reoxygenation. Carnosine inhibited by 60% the lactate dehydrogenase release from cardiac cells. A conclusion is drawn that the protective effect of carnosine is due to its membrane-stabilizing action which is implemented during inhibition of peroxidation of membrane lipids.