The bronchial hyperreactivity characteristic of asthma may be related to enhanced parasympathetic nervous activity. We postulated that an abnormality in the autonomic control of airway caliber might be reflected by a parallel change in the reflex control of heart rate. We examined the heart-rate variations induced by deep breathing (respiratory sinus arrhythmia), the Valsalva maneuver, and standing up from the recumbent position in asthmatic subjects and nonasthmatic control subjects. The asthmatic patients had evidence of enhanced parasympathetic neural drive to the sinoatrial node, as manifested by a significantly greater magnitude of respiratory sinus arrhythmia, than the controls (p less than 0.0005). We were unable to induce a similar change in normal subjects by resistance breathing. A statistical analysis suggested the presence of a relationship between the magnitude of respiratory sinus arrhythmia and the degree of bronchial hyperreactivity in a group of asthmatic patients. Our results support the hypothesis that enhanced parasympathetic activity is an important factor in the pathogenesis of bronchial asthma.