It is known that the cardiac necroses produced by 9α-fluorocortisol (F-COL) plus Na2HPO4are prevented by the administration of chlorides or potassium salts. In this paper it is shown that various chlorides not only significantly correct the depletion of myocardial potassium elicited by F-COL + Na2HPO4but increase the potassium concentration in the heart of normal animals.In view of these facts and of earlier experiments that have shown that F-COL alone diminishes cardiac potassium without producing necrosis, it is concluded that the depletion of potassium is necessary, but not sufficient, for the development of infarctoid cardiopathy and that the protective electrolytes (chlorides and potassium salts) act through the correction of this abnormality of the potassium level in the heart.