Ion transport in the intestine of Gobius niger, a euryhaline teleost, was studied in both isotonic and hypotonic conditions. Isolated tissues, mounted in Ussing chambers and bilaterally perfused with isotonic Ringer solution, developed a serosa negative transepithelial voltage and a short circuit current indicating a net negative current in absorptive direction. Bilateral removal of Cl ร and Na รพ from the bathing solutions as well as the luminal removal of K รพ in the presence of Ba 2รพ (10 ร3 M) almost abolished both V t and I sc . Similar results were obtained by adding bumetanide (10 ร5 M) to the luminal bath while other inhibitors of Cl ร transport mechanisms were ineffective. These observations suggest that salt absorption begins with a coupled entry of Na รพ , Cl ร , and K รพ across the apical membrane; a Ba 2รพ inhibitable K รพ conductance, demonstrated also by micropuncture experiments, recycles the ion into the lumen. Salt entry into the cell is driven by the operation of the basolateral Na รพ /K รพ -ATPase since serosal ouabain (10 ร4 M) completely abolished both V t and I sc ; this pump also completes the Na รพ absorption. The inhibitory effect of both serosal bumetanide (10 ร4 M) and SITS (5?10 ร4 M) suggests that Cl ร would leave the cell via the KCl cotransport, the Cl/HCO 3 ร antiport and/or conductive pathways. Bilateral exposure of tissues to hypotonic media produced a reduction of both the transepithelial voltage and the short circuit current probably due to the activation of homeostatic ionic fluxes involved in cell volume regulation. The results of experiments with both isolated enterocytes and intestine exposed to hypotonic solution suggested that the recovery of cell volume, after the initial cell swelling, involves a parallel opening of K รพ and Cl ร channels to facilitate net solute and water effluxes from the cell.