Adolescent anorexia nervosa, a psychiatric disease with high mortality, is often associated with bradycardia. We studied the vagal control of sinus node function in anorexic subjects, to investigate the mechanism of anorexic bradycardia. Cardiac vagal tone was determined in a group of 11 adolescent anorexic girls and in 11 age- and height-matched controls. Cardiac vagal tone in the anorexic patients was measured as the change in R-R interval in response to complete cholinergic blockade; in addition, non-invasive indices of cardiac vagal tone and baroreflex sensitivity were determined in both anorexic and control subjects. Cardiac vagal tone in anorexic subjects was 465 +/- 52 (SE) ms, about 30% higher than values reported for healthy subjects. Vagal tone values were directly related to percent weight loss (R = 0.69, P = 0.017). Non-invasive indices of both cardiac vagal activity and baroreflex sensitivity were significantly higher in the anorexic group as compared to controls; the percent increase of cardiac vagal tone, however, exceeded the increase of baroreflex sensitivity. Cardiac vagal hyperactivity significantly contributes to the bradycardia of anorexic subjects. The excess vagal activity is only partly explained by enhanced baroreflex sensitivity.