AbstractGastric acid secretion was measured in unanaesthetized codfish surgically equipped with a catheter draining the stomach. The pylorus was ligated. Without perfusion of the intestine, or during perfusion with pure sea water (100% SW) or slightly diluted (67%) SW, gastric acid secretion was low (<10 μmol H+/kg‐h) and the fishes became dehydrated during the experiment. Perfusion of the intestine with 50% or 33% SW enabled the fishes to compensate for the water loss to the environment and greatly enhanced gastric acid secretion. Acid secretion was elevated also in fishes in which the dehydration was prevented by an intramuscular infusion of saline. Perfusion of the intestine (8.5 ml/h) decreased the hourly effluent volume (‘drinking rate’) from the stomach to almost zero within 2 h. The effect was independent of the degree of dilution of the perfusing SW, but was less pronounced when the perfusion rate was lowered. Intra‐muscular saline also depressed ‘drinking rate’. The acid secretion prevailing during perfusion of the intestine with SW at a dilution preventing dehydration–suggested to represent the basal acid secretion of the codfish– was inhibited by atropine, hexamethoniu