The present study was performed on conscious, chronically instrumented dogs, which underwent selective blockade of sympathetic adrenergic and vagal outflows. Excitements were performed on these animals in normal states (N), after chronic treatment with guanethidine, for sympathetic blockade (SB), after cold vagal blockade (VB), and after combined sympathetic and vagal blockade (SB + VB). Heart rate and arterial blood pressure were monitored in all the experiments, while a group of dogs was also tested with an electromagnetic flowmeter on the superior mesenteric artery. The role of the sympathetic and parasympathetic controls in the defence reaction was assumed from comparison of experiments performed in the presence or in the absence of each (or both) autonomic component(s). In the SB + VB condition, excitement was followed by sudden hypotension, without changes in heart rate. In VB experiments, a brief and transient hypotension appeared, followed by gradually developed hypertension, while heart rate progressively rose in about 5 s; there was no sudden increase in mesenteric vascular resistance, which contrasted with the very marked reaction in N experiments. Under vagal control alone (SB), the stimulus elicited prompt tachycardia and hypertension, followed by a period of moderately reduced blood pressure. We conclude that, while the defence reaction leads to a sudden fall of arterial blood pressure, in the absence of compensatory mechanisms, both branches of the autonomic nervous system play a protective role against hypotension. In addition, the modulation of the vagal outflow, leading to sudden changes in the heart performance, seems to be responsible for the initiation of the overall haemodynamic adjustments following excitements. The possibility that withdrawal of the parasympathetic outflow to the heart may raise arterial blood pressure was verified in a special experiment in which artificial vagal stimulation in a conscious (vagal blocked) dog, was turned off for brief periods, before and after guanethidine. This led to changes in blood pressure and heart rate very similar to those seen at the onset of the defence reaction, both in N and SB conditions.