It is reported on clinical and haemodynamic results with Bonnecor, a new antiarrhythmic drug with class I and IV properties, on 68 test persons in parenteral and oral administration. The haemodynamic acute investigations (dosage 2 x 0.1 to 1 x 0.3 mg/kg body weight) which were performed on 10 test persons with compensated heart diseases did not show any negative haemodynamic effects of Bonnecor at rest and on exercition. The chronic effect on ventricular extrasystoles of the degrees of severity mostly II-IV were investigated in the open experiment on 17 patients as well as on 3 patients with supraventricular dysrhythmias and on 30 patients with ventricular extrasystoles Lown III-IV in the single blind cross-over experiments to verapamil retard. A clinically relevant result with regard to the reduction of the ventricular extrasystole about 75% was found in 23% of the patients (open study) and 33.3% of the patients (cross-over), respectively. The reduction of the ventricular extrasystole was significant compared with the empty value. Under the influence of verapamil a significant reduction of the ventricular extrasystole could not be verified, the differences between Verapamil and Bonnecor were, however, not to be ascertained statistically. The effect of Bonnecor on couplets and volleys was clinically relevant in 52 and 46%, respectively, of the patients (reduction greater than or equal to 93%) and approximately corresponds to the results with other antiarrhythmic drugs. The tolerability is to be estimated as relatively good.(ABSTRACT TRUNCATED AT 250 WORDS)