The importance of salt and water absorption from the food by the gastrointestinal tract and its impact on iono- and osmoregulation have been largely overlooked by fish physiologists. The present review aims to correct this situation. Techniques for overcoming the practical difficulties of studying ionoregulation in feeding fish are critically assessed, and ion and water contents of a range of diets are surveyed. In freshwater fish, the quantities of most major electrolytes ingested via a normal ration far exceed those transported from the water by the gills, but net absorption rates of specific ions vary greatly with a range of influences, including complex interactions involving mucins and bile salts. The stomach plays a key role in absorption, while net secretion generally occurs in the anterior intestinal region due to biliary and pancreatic discharge. Dry commercial diets help minimize water uptake, while high NaCl diets have marked effects on plasma composition, branchial ion fluxes, and renal function. In seawater teleosts, Na+, K+, Cl- and water absorption from the food are superimposed on ion and water transport from drinking; Ca2+ and Mg2+ are largely excluded. Effects of feeding on the ionoregulatory functions of gills and kidney remain to be investigated. Marine elasmobranchs constitute a special case due to their urea-based osmoregulary strategy; feeding causes marked stimulation of urea synthesis, urea secretion into the chyme, and rectal gland metabolism. Future directions are highlighted.