A study was undertaken to examine 55 (27 females and 28 males) patients aged 25 to 74 years who had ventricular arrhythmias. Twenty five patients were diagnosed as having mitral valvular disease concurrent with predominant stenosis, 30 presented with prevalent mitral dysfunction. Twenty eight patients showed Stages I to IIA circulatory failure, and 27 had Stages IIB to III heart failure. All the patients displayed perpetual ciliary arrhythmia of various duration. More frequent and severe ventricular arrhythmias were recorded in mitral valvular disease patients with predominant mitral dysfunction than in those with stenoses. When the plasma digoxin concentration was less than 1.1 ng/ml in patients with a low end-diastolic volume and initial signs of circulatory failure, the agent produced an antiarrhythmic effects on ventricular arrhythmias in many cases, in mitral valvular disease patients with predominant stenosis in particular. The arrhythmogenic effect of digoxin was found in 47.6% patients with prevalent mitral dysfunction concurrent with Stages IIB-III circulatory failure. The agent may show arrhythmogenic action in mitral valvular disease patients with prevalent mitral dysfunction who had larger cardiac volumes and plasma digoxin concentrations of no more than 1.6 ng/ml in the absence of clinical signs of digitalis intoxication.