The literature on the effect of some of the inhalation anesthetic agents on the tone of the heart muscle was reviewed briefly.The effects of these agents were studied in the dog heart-lung preparation.In this preparation chloroform causes rapid dilatation with early rises in pulmonary and auricular pressure.Ether causes marked dilatation even in subanesthetic levels. Beginning with blood levels corresponding to second plane anesthesia this dilatation exceeds the physiologic as indicated by auricular and pulmonary pressure rises.Cyclopropane in anesthetic range causes a definite dilatation, but not so marked as that produced by ether.Divinyl ether was not studied quantitatively, but apparently causes a degree of dilatation greater than that produced by cyclopropane, but not nearly so great as that produced by ether.Nitrous oxide causes slight cardiac dilatation even though administered with an adequate amount of oxygen.These effects are the result of impairment of the efficiency of the myocardium.