The carotid sinus baroreceptors were stimulated in 34 patients with heart failure (NYHA II-III) and sinus rhythm. We found that reflex bradycardia is caused by a selective increase in parasympathetic activity and the increase in cycle length depends on the degree of baroreceptor stimulation, therefore a regression line could be calculated for each patient. The slopes of these regression lines were used as an index of baroreflex sensitivity. Initial heart rate, age, stroke volume index, and plasma-noradrenaline concentration correlated with baroreflex sensitivity. However, according to multiple regression analysis (r = 0.64, p less than 0.001), only age correlated negatively (p less than 0.05), and the stroke volume index correlated positively (p less than 0.001) with baroreflex sensitivity: the lower the stroke volume index the smaller the baroreflex sensitivity. This indicates that reflex increases in parasympathetic activity depend on the degree of heart failure.