In 14 patients with heart failure (New York Heart Association class 2-3) and sinus rhythm the carotid sinus baroreceptors were stimulated to induce a reflex mediated decrease of sympathetic efferent activity and a simultaneous increase in vagal tone. Five patients were in severe heart failure (New York Heart Association class 3) with raised plasma concentrations of noradrenaline at rest (2.99 (0.86) nmol/l (mean (SD)) and nine patients had less severe heart failure (class 2.2 (0.2)) and normal plasma concentrations of noradrenaline at rest. The haemodynamic responses during arterial baroreceptor stimulation were different in both groups. In all five patients with severe heart failure cardiac output increased whereas in the nine patients with less severe heart failure it was unchanged or decreased. The increase of cardiac output in the group with severe heart failure was solely the result of a significant increase of stroke volume index (by 9 (2) ml/m2). In the nine patients with less severe heart failure stroke volume remained unchanged but heart rate decreased significantly by 7 (2) beats/min during baroreceptor stimulation. These data show that an integrated change of autonomic activity consisting of a decrease in sympathetic tone and an increase in vagal activity leads to an increase of stroke volume in patients with severe heart failure and hence to haemodynamic improvement.