Withdrawal of the efferent vagal tone to the heart is an important factor of the increase of cardiac output (CO) and arterial blood pressure (ABP) in several conditions, such as exercise, emotion, postural changes. Vagal withdrawal enhances cardiovascular performance both by increasing heart rate (HR) and by other mechanisms, which were globally named 'non-chronotropic mechanisms'. The nature of these non-chronotropic mechanisms was studied in open-chest dogs under morphine-chloralose anesthesia. After cutting the cervical vagi and all the branches of the stellate ganglia except for the ansae subclaviae, the animals were prepared for recording HR, ABP, CO and left ventricular pressure (LVP). The experiments started during control vagal stimulations and consisted either in turning the vagal stimulators off (STOP), or in raising HR by atrial placing without withdrawing vagal stimulation (PACE), or in turning the vagal stimulators off while keeping HR constant by atrial pacing since the control vagal stimulation (STPA). Thus, STOP, PACE and STPA produced withdrawal of all vagal effects, of the chronotropic effects and of the non-chronotropic effects, respectively. Non-chronotropic mechanisms were evaluated both as the effects of STPA and as the difference between the effects of STOP and PACE. Experiments were repeated during stellate ganglion stimulation and during simultaneous atrio-ventricular pacing, to evaluate the role of vagosympathetic interactions and of atrial contractility. CO increased by 25% after STOP, by 20% after PACE and by 5% after STPA in the absence of sympathetic stimulation and by 30% after STOP, by 20% after PACE and by 10% after STPA during sympathetic stimulation. Stellate ganglion stimulation doubled non-chronotropic effects probably by potentiating vagal effects on myocardial contractility: after STPA the maximum LVdP/dt increased by 2% without sympathetic stimulation and by 7% with sympathetic stimulation. In all conditions, the increases in ABP after STOP, PACE and STPA were small and not statistically different between STOP and PACE. Simultaneous atrio-ventricular pacing in the absence of sympathetic stimulation nearly abolished non-chronotropic mechanisms, since CO increased to about the same extent both with STOP and with PACE. It is concluded that non-chronotropic mechanisms on vagal withdrawal consist mainly in the enhancement of atrial contractility and in the release of vagal restraint on the sympathetic effects upon the ventricles.