1. Recent advances in knowledge of intestinal physiology have provided some insight into disturbed mechanisms and their clinical effects; for example, diarrhoea can now be defined biochemically as excessive fluid and electrolyte loss due to their malabsorption or excessive secretion. 2. Because of differences in structure and in absorptive and secretory mechanisms, the various parts of the gut perform different functions. In the jejunum, transport activity is extensive and the rapid equilibration of its content provides the optimal absorptive mixture. Functionally, the ileum and colon are similar; compared with the jejunum, they have greater absorptive capacity for electrolytes and generate significantly higher transmural electrical potentials. In the colon, some transport mechanisms are potentiated by adrenocortical steroids. 3. Water and electrolyte absorption and secretion are the end-products of bidirectional fluxes across the intestinal wall that are several times greater than net movement in either direction. Secretion is the surplus of negative flux (into the lumen) and absorption the surplus of positive flux (out of it). 4. Many electrolyte transport mechanisms require the absorption of other electrolytes or non-electrolytes, and some are concerned with electrolyte exchange. Water transport is always passive, in the direction of solute flow, but its solvent drag can move solutes across the intestinal membrane.