1. Isolated intestinal preparations from migrating Lampetra fluviatilis re-adapted to 50% sea water were used to localize ion transport.
2. A large, active, monovalent-ion flux was found in the anterior intestine and its magnitude decreased towards the posterior end of the intestine.
3. The monovalent-ion flux is responsible for a net water flux from mucosa to serosa in the anterior intestine.
4. It is tentatively suggested that both sodium and chloride are actively transported by the mucosa. The divalent ions may be carried along with the monovalent ions to a limited extent.
5. Studies at the light- and electron-microscope levels indicate that columnar cells of the anterior intestine are responsible for ion transport, and there is evidence that the same cells produce mucus.