Biobehavioral factors in Cardiac Arrhythmia Pilot Study (CAPS). Review and examination. The behavioral studies component of the multicenter Cardiac Arrhythmia Pilot Study (CAPS) was designed to examine the relation of biobehavioral factors and frequency of ventricular premature complexes (VPCs), efficacy of antiarrhythmic therapy, and disease end points in a study population that had experienced recent myocardial infarction and significant ventricular ectopy. Biobehavioral factors included both psychosocial (depression, anxiety, social support, type A behavior, mood, defensiveness, and anger expressiveness) and psychophysiological (heart rate and blood pressure reactivity to a videogame stressor) variables. Data were collected at baseline and at 3-, 6-, 9-, and 12-month follow-ups. Of the 502 patients enrolled in CAPS, 353 participated in the behavioral studies component. At baseline, assessments of psychosocial variables revealed the CAPS study population to be generally similar to other heart disease populations, and no relation between these variables and psychophysiological reactivity or arrhythmias was found. At follow-up among patients assigned to the placebo condition, biobehavioral variables were not related to levels of VPCs or VPC suppression. Cox regression analyses revealed that type B behavior, depression, and reduced heart rate reactivity were associated with increased clinical events, even after controlling for baseline left ventricular ejection fraction, myocardial infarction before the qualifying event, use of beta-blockers, use of digitalis, Q wave of qualifying myocardial infarction, and presence of unsustained ventricular tachycardia on baseline electrocardiogram. It is hypothesized that the relation among reduced heart rate reactivity, depression, and clinical events is mediated by diminished cardiac vagal tone. 