The sympatho-adrenal system is involved in the regulation of energy metabolism and responds to changes in energy intake. Energy restriction diminishes the sympathetic drive while overfeeding increases it. Chronically energy deficient subjects show an enhanced alpha and beta receptor responsiveness, which may be of a denervation type, associated with the reduced sympathetic activity that results from the decrease in dietary energy intake. The reduced basal metabolism of energy restricted subjects is partly attributable to the decrease in sympathetic activity. Chronically energy deficient individuals also show a suppressed thermogenic response to infusions of adrenergic agonists such as noradrenaline. Although sympathetic activity plays a modulating role in the energy homeostasis of the chronically energy deficient, it is not possible to explain seasonal fluctuations in energy balance.