SUMMARYExperiments on Gulf toadfish Opsanus beta demonstrate how feeding impacts osmoregulation in the marine teleost intestine. A high Ca2+diet of pilchards Sardina pilchardus ([Ca2+]=404.2 mmol kg-1) was compared to a low Ca2+ diet of common squid Loligo forbesi ([Ca2+]=1.3 mmol kg-1), as high[Ca2+] has been shown to stimulate intestinal anion exchange. Gastrointestinal fluids and blood plasma were collected over a time course from pre-feeding to 216 h post feeding. Following food intake, monovalent ions were largely absorbed across the intestinal epithelium, leaving a fluid rich in divalent ions, which have a lower osmotic coefficient and effectively reduce osmotic pressure in the lumen to allow for enhanced fluid absorption. Concentrations of Cl- and HCO -3 in fluid along the gastrointestinal tract of fish fed both diets, particularly 1 and 2 days post-feeding, demonstrate that apical Cl-/HCO -3 exchange plays a vital role in postprandial Cl- and water absorption. Postprandial acid-base balance disturbance as indicated by plasma alkalinization was limited or absent,indicating compensation for gastric acid secretion in this teleost fish. Plasma osmolality peaked 12 h post-feeding in toadfish fed squid, but was not accompanied by a significant increase in inorganic ion concentrations. Transient fluid secretion by the gastrointestinal tract was evident from reduced luminal Mg2+ and SO 2-4concentrations for 24-48 h post feeding. Discrepancy between the sum of inorganic osmolytes and measured osmotic pressure was attributed to organic osmolytes, which occurred at high concentrations in the stomach and anterior intestine for up to 24 h post feeding.