Using His bundle electrograms and the ventricular extrastimulus technique (single premature stimulus during sinus rhythm - S1 method; single premature stimulus during ventricular pacing - S1 S2 method; and two premature stimuli during ventricular pacing - S1 S2 S3 method) the occurrence of repetitive ventricular responses was tested in 51 patients. Thirty-two of 51 patients had organic heart disease and 19 of 51 patients (37%) had no evidence of heart disease. No patient had spontaneous or exercise-induced ventricular tachycardia, sudden death, or a recent myocardial infarction (less than six months). Repetitive ventricular responses were induced in 38 of 51 patients. In 33 of 51 patients (65%) the repetitive ventricular responses were reproducible. In 26 patients (51%), they were caused by local re-entry, and in 28 patients (55%) by bundle-branch re-entry. In addition, 16 patients had reproducible repetitive ventricular responses resulting from both bundle-branch and local re-entry. Repetitive ventricular responses caused by local re-entry were induced by the S1 method in only one patient (4%), by the S1 S2 method in seven of 26 patients (27%), and by the S1 S2 S3 method in 24 of 26 patients (92%) and were reproducible in 86% of patients. The incidence of repetitive ventricular response caused by local re-entry was significantly higher in patients with organic heart disease versus those without organic heart disease. Repetitive ventricular responses caused by bundle-branch re-entry were induced in only one patient (2.5%) by the S1 method, in 21 of 28 patients (75%) by the S1 S2 method, and in 12 of 28 patients (42%) by the S1 S2 S3 method. There was no significant difference between the occurrence of repetitive ventricular responses resulting from bundle-branch re-entry and the presence or absence of organic heart disease. All patients without repetitive ventricular responses have been followed for six to 17 months (average = 12 months) with 24 hour ambulatory electrocardiographic recordings. None of the patients with repetitive ventricular responses caused by local re-entry and bundle-branch re-entry has developed ventricular tachycardia and/or sudden cardiac death. Repetitive ventricular responses caused by local re-entry can be indued in a significant number of patients with organic of repetitive ventricular responses caused by local re-entry whereas the S1 S2 method is the most sensitive for the induction of repetitive ventricular responses caused by bundle-branch re-entry. The S1 method is the least sensitive for the induction of both local re-entry and bundle-branch re-entry. Though repetitive ventricular responses caused by local re-entry may suggest electrical instability, our follow-up studies disclosed that it is not a predictor of sudden death.