These investigations address the problem of sudden cardiac death. The three objectives include development of methods for identifying the subject at increased risk, definition of predisposing mechanisms, and introduction of measures for protecting against ventricular fibrillation (VF). Both animal and clinical investigations are to be carried out. Animal studies have the following three aims: (1) to examine the role of coronary vasospasm on ventricular vulnerability to VF, specifically probing the effect of various ischemic states, and the contribution of platelets, prostaglandins and neural activity; (2) to evolve methods for sensitizing the heart and thereby expose myocardial electrical instability; and (3) to determine the possible value of the protective zone and nonsustained VF as new prophylactic approaches. Clinical studies will pursue four aims: (1) to determine whether the repetitive ventricular response (RVR) can serve as a method for detecting myocardial electrical instability in man; (2) to assess the value of precordial thumping in exposing susceptibility to arrhythmias; (3) to identify the site of ventricular premature beats (VPBs) by radioventriculography; and (4) to determine the role of hypokalemia in the genesis of VPBs in hypertensive patients.