Effects of niflumic acid (NFA) and flufenamic acid (FFA), the two nonsteroid anti-inflammatory agents recently reported to inhibit Cl- current in Xenopus oocytes, were examined in cultured monolayers of dog and cow trachea. Both agents showed potent inhibition to the short-circuit current (Isc), an index of magnitude of transepithelial Cl- secretion, with values of Ki of 0.02 (for NFA) and 0.06 (for FFA) mM, respectively. The sensitivity sequence of Isc to the Cl- channel inhibitors tested was NFA > FFA > diphenylamine-2-carboxylate (DPC) >> anthracene-9-carboxylate (A9C). Thus, NFA and FFA are so far the most potent commercially available Cl- channel inhibitors tested in Cl(-)-secreting epithelia. The sensitivity sequence of 36Cl uptake to the above Cl- channel inhibitors in Xenopus laevis oocytes was found to be identical to the cultures of trachea. This seems to imply that the membrane Cl- channels of Xenopus oocytes are functionally similar to that identified in mammalian Cl(-)-secreting epithelia.