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The data emerging from the work we have been doing on the role of the brain In the regulation of Na^ bomeostasis is interesting raw material for analysis in a symposium on control systems.It is clear from many facets of contemporary physiological knowledge, including comparative studies, that the evolutionary progression of life from the oceans and estuaries and swamps to the free ranging land species has involved development of a complex organization of Biic control of salt and water.The control of sodium homeostasis involves regulation of input and output. On the intake side, the processes to be considered are:(1) The innate behavioural mechanisms of specific salt appetite drive generated by onset of body Na+ deficit. (2) The hedonistic palatal)ility determined salt ingestion-i.e., intake without any metabolic requirement occurring because the creature likes tlie taste. (3) The possibility of central nervous processes causing aversion to suit in tbe instanee of body overload-i.e.. processes over and above the recognized taste aversion normally produced by salt solutions of high concentrations. Output concerns (i) tlie regulation of aldosterone secretion (ii) the operation of mechanisms which in certain circumstances amplify the action of aldosterone on target organs such as occurs in Na deficiency, (iii) natriuretic processes, physical and possibly hormonal, which operate in the face of body surfeit.In this paper we are going to confine attention to the appetite element of intake. However, it is important to bave instanced above the several components of control. These systems luu'e emerged concurrently during phylogenesis as a result of the various environmental stresses upon Nu^ homeostasis. Whereas it is formally possible that the control mechanisms are largely independent of one

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