In 15 patients with ventricular arrhythmias without previous treatment the effectiveness of 2-amino-2',6'-dimethylpropionanilide (tocainide, Xylotocan) and propafenone was compared in a cross-over-trial. 14 Patients had ventricular arrhythmias of the Lown classification IVa-V, 1 patient of Lown classification III. In 3 patients after the first course of tocainide resp. propafenone no change in therapy was made because of reducing the Lown classification by two or more grades. All patients had 24-h-ECG-Holter-Monitoring before, at the 4th day before change of therapy, and at the 9th day of therapy. Following relevant results were found: Both tocainide (p less than 0,005) and propafenone (p less than 0,05) reduce the total amount of ventriculare premature beats (VPB) without statistical significant difference between both substances. Both tocainide (p less than 0,005) and propafenone (p less than 0,02) reduce the number of isolated polytope VPB without statistical significant difference between both substances. Only tocainide (p less than 0,005) reduces couplets, but there was no statistical significant difference to propafenone. Neither tocainide nor propafenone alone was able to make total suppression of the ventricular arrhythmias in the patients included in this study. Both substances were able to lower VPB buth with a more favourable but statistically not significant therapeutic effect of tocainide.