Angiotensin-converting enzyme (ACE) inhibitors reduce mortality and morbidity in patients with congestive heart failure. These effects may be mediated, at least in part, by suppression of lethal ventricular tachycardia (VT). The aims of this study were to examine whether the ACE inhibitor cilazapril reduces ventricular arrhythmia in patients with congestive heart failure and, if cilazapril does reduce ventricular arrhythmia, to determine whether this reduction is associated with suppression of sympathetic nerve activity in these patients. Thirty-two congestive heart failure patients (left ventricular ejection fraction, 35 +/- 6%; New York Heart Association class II or III) with VTs (Lown grade IVa or IVb) were randomly assigned to receive either conventional therapy, consisting of diuretics and digitalis (control group), or conventional therapy plus cilazapril (cilazapril group). Twenty-four-hour ambulatory electrocardiographic monitoring was performed at baseline and after 2 months of therapy. Plasma norepinephrine levels and heart rate variability (standard deviation about the mean RR interval) were compared at baseline and after 2 months of treatment. The control group demonstrated no significant change in arrhythmia frequency after 2 months of treatment. In the cilazapril group, however, the number of ventricular couplets and VT runs was significantly decreased. In association with this reduction, plasma norepinephrine levels were decreased, and heart rate variability was increased. These results suggest that cilazapril has antiarrhythmic effects, which may be produced by suppressing high sympathetic activity, in patients with congestive heart failure. It should be noted that the study group was small and that, although ventricular dysrhythmia was reduced with therapy, it remained substantial. Further study is needed to verify these results and to determine the exact causes of the reduction.