20 patients with idiopathic complex ventricular arrhythmias received propafenone 450 mg/d, disopyramide 600 mg/d and metoprolol 100 mg/d. Before commencement of the 3-week treatment period the 95% normal range for spontaneous changes in ventricular extrasystoles (VES), as couplets and runs, were determined from three 24 h-ECG recordings in each patient under drug-free conditions. A drug effect was assumed when the rate of VES/24 h for the control period decreased by greater than or equal to 83.5% or increased by greater than or equal to 505% in the test period. The frequency dependent normal range for couplets varied between a decrease from 100% to 92% (greater than or equal to 16 to greater than or equal to 44/d) and an increase from 650% to 1000% (greater than or equal to 3 to greater than or equal to 38/d) and for runs between a decrease from 100% to 85% (greater than or equal to 6 to 32/d) and an increase from 600% to 1000% (greater than or equal to 1 to 14/d). A decrease in all rhythm disturbances under the action of the 3 drugs could be shown for the whole group (P less than 0.01). On the basis of the calculated normal range, ventricular extrasystoles in the patients decreased significantly by 26-37%, couplets by 13-33% and runs by 0-55% depending to the drug. A drug dependent arrhythmogenic effect occurred in 4 patients. A preference for one or other of the drugs could not be established statistically.