Impaired reflex control of heart rate seems to be associated with increased risk of sudden cardiac death. To assess the effect of aortic valve stenosis on parasympathetic heart rate control we measured the heart rate responses to deep breathing and to standing up, non-invasive measures of cardiac parasympathetic activity, in 24 patients with valvular aortic stenosis and in 24 healthy asymptomatic subjects of the same age group. All but one of the patients were symptomatic and the mean aortic valve pressure gradients ranged from 32 to 114 mmHg. The heart rate response to deep breathing was significantly (P less than 0.05) lower in valve patients than in the healthy subjects. The heart rate changes evoked by standing up, however, did not differ significantly between the groups. The heart rate response to deep breathing was inversely weakly related to the left ventricular end-diastolic pressure (r = -0.41, P less than 0.05), but not significantly to the aortic valve pressure gradient, valve area, left ventricular ejection fraction or presence of coronary artery disease. Our results suggest that the impairment of reflex heart rate control is common in patients with significant aortic valve stenosis.