Na+ absorption across Aplysia gut was mediated by a Na+/K+-ATPase located in the enterocyte basolateral membrane. 2. In the absence of Na+ in the bathing medium, net Cl- absorption across Aplysia gut wall was identical to the SCC. 3. Intracellular Cl- was at a lower electrochemical potential in Aplysia enterocytes than in either the mucosal or serosal medium. 4. Cl--stimulated ATPase activity was localized in the basolateral membrane of Aplysia enterocytes. 5. ATP-dependent Cl- transport was localized in the basolateral membrane of Aplysia enterocytes. 6. In Aplysia gut primary active transport systems for both Na+ and Cl- are postulated based on the evidence presented.
