The consequences of feeding and digestion on acid–base balance and regulation in a marine teleost (seawater-acclimated steelhead trout; Oncorhynchus mykiss) were investigated by tracking changes in blood pH and [HCO3–], as well as alterations in net acid or base excretion to the water following feeding. Additionally the role of the intestine in the regulation of acid–base balance during feeding was investigated with an in vitro gut sac technique. Feeding did not affect plasma glucose or urea concentrations, however, total plasma ammonia rose during feeding, peaking between 3 and 24 h following the ingestion of a meal, three-fold above resting control values (∼300 μmol ml–1). This increase in plasma ammonia was accompanied by an increase in net ammonia flux to the water (∼twofold higher in fed fish versus unfed fish). The arterial blood also became alkaline with increases in pH and plasma [HCO3–] between 3 and 12 h following feeding, representing the first measurement of an alkaline tide in a marine teleost. There was no evidence of respiratory compensation for the measured metabolic alkalosis, as PaCO2 remained unchanged throughout the post-feeding period. However, in contrast to an earlier study on freshwater-acclimated trout, fed fish did not exhibit a compensating increase in net base excretion, but rather took in additional base from the external seawater, amounting to ∼8490 μmol kg–1 over 48 h. In vitro experiments suggest that at least a portion of the alkaline tide was eliminated through increased HCO3– secretion coupled to Cl–absorption in the intestinal tract. This did not occur in the intestine of freshwater-acclimated trout. The marked effects of the external salinity(seawater versus freshwater) on different post-feeding patterns of acid–base balance are discussed.