The actions of norepinephrine and isopropyl norepinephrine on the rhythmic property of the heart were studied and compared with that of epinephrine. Norepinephrine does not abolish cardiac standstill by carotid sinus stimulation, has no effect on the ventricular rate of heart block and induces a sinus bradycardia. Isopropyl norepinephrine acts similarly to epinephrine in that it abolishes the induced cardiac standstill, increases the ventricular rate of heart block and produces a sinus tachycardia. In the treatment of cardiac arrest, the isopropyl compound appears to possess optimum features as it is very potent in the prevention of cardiac standstill and does not predispose to ventricular fibrillation.