Cardiac electrophysiological studies will be used in patients with symptomatic ischemic heart disease to identify ischemia-induced cardiac electrical instability. Conduction properties and refractory periods of the atria, AV node, fascicles and ventricles will be determined by the extrastimulus technique both before and after ischemia is induced by atrial pacing in patients with angina pectoris. Similar studies will be performed in patients after myocardial infarction, but at lower paced heart rates. Coronary angiography and left ventriculography will then be performed in both groups of patients. Induction of repetitive ventricular reponses to single ventricular stimuli either before or after induction of ischemia by right atrial pacing will identify a group of patients at high risk of developing ventricular arrhythmias which may lead to sudden death. Production of atrioventricular block or fascicular block with induction of ischemia after rapid atrial pacing will identify patients who may be prone to develop permanent fascicular or atrioventricular block if they later experience myocardial infarction. Patients will be followed prospectively to determine the natural history of the abnormalities induced by atrial pacing. Correlations will be made with the coronary anatomy and left ventricular function. Identification of high risk groups in this manner should allow later prospective trials of interventions designed to alter the risk of sudden death.