FOLLOWING ITS INTRODUCTION into the practice of cardiology in 1972, 1 programmed electrical stimulation (PES) of the heart has developed from a technique used for diagnostic purposes into one used for clinical management of patients with life-threaten-ing ventricular arrhythmias. During the same years we have seen a gradual change from relatively simple pacing protocols using one or two prematurestimuli during ventricular stimulation to" aggressive" protocols using up to four pre-mature stimuli, many basic pacing rates, and different sites of ventricular stimulation. During PES some factors can, but many cannot, be controlled by the investigator. Among those that are controllable are (1) stimulus (strength, duration, type of current, number and rate of basic stimuli, number and interval of premature stimuli),(2) stimulation site,(3) mode of stimulation (unipolar or bipolar), and (4) interelectrode distance. Some factors that arenot con-trollable are (1) type of spontaneous tachycardia,(2) etiology of spontaneous tachycardia,(3) resting heart rate,(4) autonomic state,(5) electrophysiologic properties of the arrhythmia substrate,(6) autonomic re-sponse to pacing or the administration of drugs, and (7) hemodynamic and ischemic consequences of pacing and drug administration. In our opinion a discussion on the value of PES in the study and treatment of life-threatening ventricular arrhythmias and the importance