Marine teleosts generally secrete basic equivalents (HCO3–) and take up Na+ and Cl– in the intestine so as to promote absorption of H2O. However, neither the integration of these functions with feeding nor the potential role of the gut in ionoregulation and acid–base balance in freshwater have been well studied. The euryhaline killifish (Fundulus heteroclitus) is unusual in lacking both an acid-secreting stomach and a mechanism for Cl– uptake at the gills in freshwater. Responses to a satiation meal were evaluated in both freshwater- and seawater-acclimated killifish. In intact animals, there was no change in acid or base flux to the external water after the meal, in accord with the absence of any post-prandial alkaline tide in the blood. Indeed, freshwater animals exhibited a post-prandial metabolic acidosis (‘acidic tide’), whereas seawater animals showed no change in blood acid–base status. In vitro gut sac experiments revealed a substantially higher rate of Cl– absorption by the intestine in freshwater killifish, which was greatest at 1–3 h after feeding. The Cl– concentration of the absorbate was higher in preparations from freshwater animals than from seawater killifish and increased with fasting. Surprisingly, net basic equivalent secretion rates were also much higher in preparations from freshwater animals, in accord with the ‘acidic tide’; in seawater preparations, they were lowest after feeding and increased with fasting. Bafilomycin (1 μmol l–1) promoted an 80% increase in net base secretion rates, as well as in Cl– and fluid absorption, at 1–3 h post-feeding in seawater preparations only, explaining the difference between freshwater and seawater fish. Preparations from seawater animals at 1–3 h post-feeding also acidified the mucosal saline, and this effect was associated with a marked rise in PCO2, which was attenuated by bafilomycin. Measurements of chyme pH from intact animals confirmed that intestinal fluid (chyme) pH and basic equivalent concentration were lowest after feeding in seawater killifish, whereas PCO2 was greatly elevated (80–95 Torr) in chyme from both seawater and freshwater animals but declined to lower levels (13 Torr) after 1–2 weeks fasting. There were no differences in pH, PCO2 or the concentrations of basic equivalents in intestinal fluid from seawater versus freshwater animals at 12–24 h or 1–2 weeks post-feeding. The results are interpreted in terms of the absence of gastric HCl secretion, the limitations of the gills for acid–base balance and Cl– transport, and therefore the need for intestinal Cl– uptake in freshwater killifish, and the potential for O2 release from the mucosal blood flow by the high PCO2 in the intestinal fluids. At least in seawater killifish, H+-ATPase running in parallel to HCO3–:Cl– exchange in the apical membranes of teleost enterocytes might reduce net base secretion and explain the high PCO2 in the chyme after feeding.