Diprafenone is a new antiarrhythmic agent currently under clinical investigation, with close chemical similarity to propafenone. In this study, the electrophysiological and haemodynamic effects of the compound were investigated both in animals, by experiment, and in man. Diprafenone produces a dose-dependent prolongation of conduction in all parts of the conducting system. Lengthening of PQ-time is more pronounced than prolongation of QRS. The atrial and ventricular refractory periods are also significantly prolonged. There are no significant changes in the QT-time. Heart rate and aortic pressure are slightly decreased. The electrophysiological and haemodynamic profile of Diprafenone is similar to propafenone with respect to a dominant local anaesthetic activity and an additional beta-sympatholytic effect. However, with respect to the dose needed, the efficacy of the new drug appears to be significantly stronger. Diprafenone can be considered an effective antiarrhythmic drug for the treatment of supraventricular and ventricular tachyarrhythmias.