Ten patients with persistent ventricular arrhythmia were studied in a comparison of the antiarrhythmic efficacy of N-acetylprocainamide (NAPA) and procainamide (PA). Each patient performed three exercise tests for 40 min., on different days, with submaximal and fixed work loads. During the first exercise test no drug was administered. During the following two tests PA and NAPA, respectively, were administered by intravenous infusion. The electrocardiogram was continously recorded and was analyzed minute by minute. Blood samples for determination of plasma drug concentration were frequently collected. Exercise alone did not significantly change the incidence of arrhythmia. Both PA and NAPA showed a similar and significant antiarrhythmic effect. A blood pressure fall was seen in two patients after administration of each drug. No other adverse reaction was observed.