Noradrenaline and adrenaline are catecholamines that play major roles in regulation of the ‘inner world’ of the body by the brain. Noradrenaline (synonymous with norepinephrine), the main neurotransmitter of the sympathetic nervous system, is responsible for tonic and reflexive changes in cardiovascular tone. Adrenaline is a key determinant of responses to metabolic or global challenges to homeostasis, such as glucoprivation(increases expression of neuropeptide Y mRNA in hindbrain neurons that innervate the hypothalamus) and of manifestations of emotional distress. In contrast with the view that the sympathetic nervous and adrenomedullary hormonal systems function as a unit (the ‘sympathoadrenal system’) to maintain homeostasis in emergencies, across a variety of situations, adrenaline responses are more closely linked to responses of the hypothalamic?pituitary?adrenocortical system than of the sympathetic nervous system. The sympathetic noradrenergic system is active even when the individual is at rest and maintains tonic levels of cardiovascular performance. Adrenoceptors in the membranes of effector cells determine the physiological and metabolic effects of catecholamines.