With the development of techniques for the qualitative and quantitative assessment of receptor function and knowledge of the biological responsiveness of neurotransmitter-mediated pathways, it is now quite clear that the response of a patient to a drug does not only involve the concentration of the drug in blood and tissue. Number and function of receptors are also important factors. A perturbation in which the receptor number is elevated is called "up-regulation", whereas "down-regulation" refers to the uncoupling between receptor and effector and the consecutive decrement in the receptor concentration. In general, there is an inverse relationship between the ambient concentration of the agonist and the number of its receptors and, therefore, the sensitivity of the target organ. The demarcation between alpha- and beta-adrenergic receptors has long been appreciated. Recent advances in the understanding of adrenergic receptors have led to the subdivision of beta-receptors; beta 1-adrenoceptors mediate the stimulation of rate and force of cardiac contraction and stimulate lipolysis. beta 2-adrenoceptors mediate smooth muscle relaxation and facilitate glycogenolysis and the release of insulin, glucagon and renin. The alpha-adrenergic receptors may also be divided into two subgroups. The alpha 1-adrenoceptors are postsynaptic located and facilitate smooth muscle constriction. Presynaptic located alpha 2-adrenoceptors mediate feedback inhibition of norepinephrine-release, while postsynaptic alpha 2-adrenoceptors facilitate smooth muscle contraction in selected vascular beds and stimulate the inhibition of various metabolic processes (insulin and renin secretion, lipolysis). The stage is now set for the application of the new knowledge of receptor function and regulation to the advancement of the practice of anaesthesia and intensive care.