The gastrointestinal tract is involved in both absorption and secretion of electrolytes and water, with absorption as the predominant process. In diarrheal diseases this balance is disturbed, and the result is net secretion. Most of the drugs used for the treatment of diarrhea at least partially act by (1) stimulating absorption only, (2) both stimulating absorption and inhibiting secretion, or (3) inhibiting secretion only. The therapeutic usefulness of an antidiarrheal agent depends on how efficiently it alters secretion and/or absorption and on how few systemic adverse reactions it causes. When more information on the regulation of absorption and secretion has been accumulated, it may be possible to develop new drugs that can be aimed directly at these processes.