Remarkable progress has been made both experimentally and clinically in defining the influence of behavioral states on susceptibility to life-threatening arrhythmias. Biological models have been developed to emulate anger and fear and have permitted detailed study of the intermediary mechanisms involved in stress-induced ischemia and ventricular fibrillation. The studies highlight the importance of adrenergic factors and the pathological significance of the poststress state. Clinically, the role of daily stresses in inducing silent myocardial ischemia and arrhythmias has been extensively characterized, and standardized behavioral stress tests have become available. Certain sleep states have been found to provoke ischemic episodes and arrhythmias. In particular, phasic rapid eye movement (REM) sleep has been shown both in animals and humans to conduce to perfusion abnormalities and propensity to fibrillation. Episodic surges in sympathetic nervous system activity appear to be the underlying basis. These conceptual and practical advances illustrate the promise of behavioral cardiology in the diagnosis and treatment of individuals at risk for sudden cardiac death.