As long as the question of which channels are responsible for cAMP-mediated epithelial Cl⁻ secretion remains unsolved, it is still important to search for specific inhibitors that might help to relate macroscopic to microscopic events. Following the report by Sheppard and Welsh (J Gen Physiol 100: 573, 1992) that glibenclamide inhibits whole-cell Cl⁻ currents in genetically manipulated fibroblasts expressing the cystic fibrosis transmembrane conductance regulator (CFTR), we have studied the effect of glibenclamide on different types of Cl⁻ channels of HT₂₉ and T84 cells at the single-channel level. Our results confirm that micromolar concentrations of glibenclamide inhibit the linear, low-conductance Cl-channel, which appears to represent CFTR and show that the inhibition results from a typical flicker block. However, the same concentrations of glibenclamide inhibit also the outwardly rectifying intermediate conductance Cl⁻ channel which, potentially, may contribute to transepithelial Cl⁻ secretion.