The antiarrhythmic efficacy of the invastigational class Ic-drug nicainoprot (N.) was evaluated in 61 patients with frequant premature ventdcular beats (PVBs) in a doubleblind, cross*over trial including four 4-day bedods with administration of placebo (PL) or N. in doses of 200, 400, 600 mg t.i.d, in randomized order. Total PVBs/24 h were significantly reduced by 400 mg N. (65 %) and 600 mg N. t.i.d. (71%). Individual therapeutic response, defined as a reduction of total PVBs > 75 %, of vantricular pairs > 90 % and elimination of ventricular runs, was found in 6 (10 %), 10(16%),21 (34 %), and 22 (36 %) patients with PL, 200 mg N., 400 mg N. and 600 mg N., respectively. Heart rate, mean blood pressure, QT and JTc intervals were not significantly influenced by any doze of N. There was a positive correlation batv~an mean plasma concentrations of N. and a mild prolongation of the PR interval. Extracardiac side effects (nausea, dizziness, gamrointastinal discomfort, exanthema) in 7 % of the patients occured mainly after 600 mg N. t.i.d. In one patient fatal ventriculer fibrillation occurred during the first treatment period (600 rng ti.d.). It is concluded that nioainoprol leads to a dose-related suppression of PVBs in patients with ventricular arrhythmias due to various cardiac disorders. With regard to both antiarrhythmic efficacy and possible adverse effects a dose of 400 mg t.i.d. appears recommendable. This dose covered more than 90 % of the responding patients with virtually no side effects.