The protocols described in this unit are used to assess the effects of new chemical entities on arrhythmias in the guinea pig. In the anesthetized guinea pig, arrhythmias are induced by a slow intravenous infusion of digoxin, which provokes extrasystoles, ventricular tachyarrhythmias, and ultimately cardiac arrest. Imipramine precipitates the occurrence of arrhythmias, whereas propranolol shows protection against them.