Synchronized precordial direct current shock has been applied in the treatment of sustained arrhythmia 84 times in 62 patients. Sinus rhythm was restored in 90% of the patients in 85% of the attempts. Single and multiple shocks were tolerated without significant electrocardiographic abnormality, aside from change in rhythm, or transaminase rise. No pulmonary or systemic emboli occurred during 71 reversions to sinus rhythm although anticoagulants had been withheld in more than half the patients. Complications were limited to the development of other arrhythmias in three patients. Synchronized direct current precordial shock is a safe, simple, and effective therapy of ectopic arrhythmia which may be utilized on an elective or emergency basis. The results are immediately apparent.