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To study possible beta-blocking effects of propafenone, a class lc antiarrhythmic agent, electrocardiographic variables were studied at rest and during exercise. Forty-two patients with symptomatic ventricular arrhythmias were included. In 31 patients, exercise electrocardiograms before and after propafenone were available for study. The mean dose of propafenone was 603 mg for all 42 patients, 585 mg for patients undergoing exercise testing (range 450–1200). At rest, the PR interval, duration of P wave and QRS complex changed significantly without any change in the resting heart rate and QT interval. During exercise there were no changes after propafenone in peak heart rate, PR interval, QT interval, duration of exercise, or blood pressure. We conclude that, while class I effects are evident both at rest and after exercise, no clinically significant beta-blocking effects of propafenone could be demonstrated either at rest or during exercise. [ABSTRACT FROM PUBLISHER]