The aim of the present study was to investigate the influence of quinidine on the effectiveness of electrical cardioversion (CV) for the reversion of supraventricular arrhythmias, the amount of electrical energy necessary for the CV and the possible complications of electrical CV. Initially, 100 CV procedures were allocated to the control group (patients free from the action of any antiarrhythmic drug), and 50 CV to the quinidine group. Quinidine was given as dihydroquinidine C1H at a dose of 500 mg/12 hours since the day before CV. In the last group, 6 (12%) patients reverted to normal sinus rhythm before electrical CV. At the time of electrical CV (100 procedures in the control group and 44 in the quinidine group), the patients who received quinidine required a lower amount of electrical energy, and showed a lower incidence of atrial premature beats as compared with the control group (11.3% versus 28%, p less than 0.05). A similar proportion of patients reverted to sinus rhythm in both groups. We conclude that the administration of quinidine before electrical CV has the following advantages: 1) 12% of patients reverted to normal sinus rhythm before electrical CV; 2) quinidine reduced the amount of electrical energy necessary for the CV, and 3) quinidine reduced the incidence of atrial premature beats after electrical CV. On the other hand, quinidine had no influence on the incidence of ventricular arrhythmias after electrical CV.