In experiments with dogs, three different methods of inducing a reversible cardiac arrest were compared: (A) the ischemic arrest for 45 min, (B) the cardiac arrest for 90 min due to injection of Cardioplegin according to KIRSCH, and (C) the cardiac arrest for 90 min due to infusion of solution LK 352 according to BRETSCHNEIDER. The body temperature was reduced to 30 degrees C during the period of cardiac arrest. From the alterations in the adenylic acid system of the left ventricular myocardium at the end of the period of myocardial standstill and after 60 min of recovery, it can be deduced that the best myocardial protection is given by method C. Method A has the least effect.