22 patients with salvoes of ventricular tachycardia were investigated electrophysiologically and pharmacologically to determine the mechanism of the arrhythmia. The appearance of the ventricular complexes was suggestive of a septal origin. The arrhythmia appeared to occur and regress within precise limits of sinus rhythm. 24 hour ECG recording and endocavitary stimulation techniques showed its essentially variable character, but occasionally a fixed coupling interval with the first extrasystole was observed, suggesting the presence of an unprotected parasystolic focus. The repetition of the extrasystolic activity, which could be triggered and terminated at will, is explained by phenomena of delayed after potentials. The results of the systematic investigation of the effects of quinidine-like drugs, beta-blockers and verapamil do not invalidate these hypotheses and show a difference of degree but not of nature between the two populations A and B of patients classified according to the quantitative character of the extrasystolic activity over 24 hours (more or less than 1000/hour). The observation of intermediate forms with simple extrasystoles and other forms with sustained tachycardia extends the significance of these results to many forms of ventricular arrhythmia.