The effect of loop diuretics on Cl transport was studied on an in vitro preparation of the bullfrog cornea. Bumetanide (10(-4) M) or furosemide (10(-3) M) added to the stromal solution decreased Cl transport measured as the short-circuit current (Isc) to values near zero. Concomitantly, transepithelial conductance (gt) decreased, whereas the intracellular potential (Vo) hyperpolarized and the fractional resistance of the apical membrane (fRo) increased. Substitution of SO4 for Cl in the tear-side solution led to prompt changes in Isc, gt, Vo, and fRo, characteristic of appreciable passive Cl movement across the apical membrane before and after inhibition. Epinephrine (10(-4) M) was similarly effective on apical membrane conductance in inhibited tissues as under control conditions, but the effective electromotive force for transepithelial Cl transport was reduced to approximately 25%. Intracellular Cl activity, measured with ion-selective microelectrodes, decreased so much that the difference in electrochemical Cl potential divided by the Faraday constant (delta mu Cl/F) was close to zero after inhibition of Isc by bumetanide. Apical Cl permeability remained essentially unchanged. Accordingly, loop diuretics inhibit Cl transport in the Cl-secreting cornea epithelium by blocking the Na-Cl symport without secondary apical effects, as believed for other Cl-reabsorbing epithelia.