Discussions of how hormones influence behavior have generally been limited to the effects of gonadal hormones on reproductive behavior and have emphasized the parsimonious arrangement whereby the same hormones involved in the biology of reproduction also influence sexual behavior. It has now become clear, however, that other hormones, in addition to their recognized influence on biological functions, can affect behavior. Specifically, peptide and steroid hormones involved in maintaining the physiological balance, or homeostasis, of body fluids also appear to play an important role in the control of water and salt consumption. The phenomenon of homeostasis in animals depends on various mechanisms that promote stability within the organism despite an inconstant external environment; the homeostasis of body fluids, whereby the osmolality (the concentration of solutes) of blood plasma is closely regulated, is achieved primarily through alterations in the intake and excretion of water and sodium, the two principal components of the fluid matrix that surrounds body cells. Appropriate compensatory responses are initiated when deviations from normal are quite small, thereby maintaining plasma osmolality within relatively narrow ranges.

In the osmoregulation of body fluids, the movement of water across cell membranes permits minor fluctuations in the concentration of solutes in extracellular fluid to be buffered by corresponding changes in the relatively larger volume of cellular water. Nevertheless, the concentration of solutes in extracellular fluid may at times become elevated or reduced by more than the allowed tolerances of one or two percent. It is then that complementary physiological and behavioral responses come into play to restore plasma osmolality to normal. Thus, for example, a decrease in plasma osmolality, such as that which occurs after the consumption of water in excess of need, leads to the excretion of surplus body water in the urine by inhibiting secretion from the pituitary gland of vasopressin, a peptide hormone that promotes water conservation in the kidneys. As might be expected, thirst also is inhibited then, to prevent further dilution of body fluids. Conversely, an increase in plasma osmolality, such as that which occurs after one eats salty foods or after body water evaporates without being replaced, stimulates the release of vasopressin, increasing the conservation of water and the excretion of solutes in urine. This process is accompanied by increased thirst, with the result of making plasma osmolality more dilute through the consumption of water. The threshold for thirst appears to be slightly higher than for vasopressin secretion, so that thirst is stimulated only after vasopressin has been released in amounts sufficient to produce maximal water retention by the kidneys—that is, only after osmotic dehydration exceeds the capacity of the animal to deal with it physiologically.