Approximatively 10 liters of fluid enter the gastrointestinal tract with food and endogenous secretions, and only less than 100 ml or 1 % leave it with the faeces. Minor changes of this equilibrium in the intestinal transport may cause diarrhoea or constipation. Functions of small and large intestine differ markedly in transport of electrolytes and water. The relatively leaky epithelium of the small intestine allows for rapid equilibrium of osmolality in both directions while the tight epithelium of the colon preserves electrolytes and water once they have been absorbed. It may compensate secretory diarrhoea of the small intestine for instance caused by bacterial toxins to a certain degree unless it is overwhelmed leading to an overflow type of diarrhoea. On the other hand, small changes of net fluid transport in the colon in either direction will lead to diarrhoea or constipation since there is no compensating mechanism behind it. Mechanisms involved in the regulation of transintestinal electrolyte and water movements are the energy providing Na⁺, K⁺-ATPase, the mediators of membrane permeability and active Cl^– secretion such as cAMP and Ca²⁺ and substances affecting the tight junctions. Various substances may affect one or more of these regulatory mechanisms. Laxatives are one of those.