In the intact organism environmental disturbances affecting the circulation often result in simultaneous changes of several groups of peripheral afferents. These elicit characteristic patterns of autonomic activity with distinctive patterns of vagal activity, of regional sympathetic neural activity and of adrenal catecholamine secretion. During simultaneous changes in several groups of afferents the autonomic responses are often non-linear, with the response to one input markedly influenced by the level of the others. When these non-linear interactions involve the central arterial baroreflex pathways the properties of the body's blood pressure system can become greatly altered. With certain combinations of afferents these interactions make it possible for the organism to better withstand large perturbations than it could do through the normal properties of the arterial baroreceptor reflex. The different neuron groups contributing to the CNS autonomic pathways release many different transmitters including noradrenaline or serotonin and changes in reflex properties result from alterations in transmitter release in one or other of the pathways of the particular network. The peripheral arterial baroreceptors become rapidly reset during sustained alterations in blood pressure. Their 'memory' for any given absolute blood pressure is only a few minutes duration. Hence sustained changes in autonomic activity depend on the properties of the CNS either through signals arising from other groups of peripheral receptors, from central 'command' or owing to changes in transmitter release in a given pathway.